7 research outputs found
Asymmetric Syntheses of Methyl <i>N</i>,<i>O</i>‑Diacetyl‑d‑3-<i>epi</i>-daunosaminide and Methyl <i>N</i>,<i>O</i>‑Diacetyl‑d‑ristosaminide
Ab initio asymmetric syntheses of
methyl <i>N</i>,<i>O</i>-diacetyl-d-3-<i>epi</i>-daunosaminide
and methyl <i>N</i>,<i>O</i>-diacetyl-d-ristosaminide, employing diastereoselective epoxidation and dihydroxylation,
respectively, of alkyl (3<i>S</i>,α<i>R</i>,<i>Z</i>)-3-[<i>N</i>-benzyl-<i>N</i>-(α-methylbenzyl)amino]hex-4-enoates as the key steps, are
reported. The requisite substrates were readily prepared using the
conjugate additions of lithium (<i>R</i>)-<i>N</i>-benzyl-<i>N</i>-(α-methylbenzyl)amide to methyl
and <i>tert</i>-butyl (<i>E</i>)-hexa-2-en-4-ynoates
followed by diastereoselective alkyne reduction. <i>syn</i>-Dihydroxylation using OsO<sub>4</sub> proceeded under steric control
on the 4<i>Re</i>,5<i>Re</i> face of the olefin
to give the corresponding diol, which subsequently underwent lactonization.
Meanwhile, epoxidation using F<sub>3</sub>CCO<sub>3</sub>H in conjunction
with F<sub>3</sub>CCO<sub>2</sub>H proceeded on the opposite 4<i>Si</i>,5<i>Si</i> face of the olefin under hydrogen-bonding
control from the in situ formed ammonium ion. Treatment of the intermediate
epoxide with concd aq H<sub>2</sub>SO<sub>4</sub> promoted highly
regioselective ring-opening (distal to the in situ formed ammonium
moiety) to give the corresponding diol (completing overall the formal <i>anti</i>-dihydroxylation of the olefin), which then underwent
lactonization under the reaction conditions. Elaboration of these
diastereoisomeric lactones through hydrogenolysis, <i>N</i>-Boc protection, reduction, methanolysis, and acetate protection
gave methyl <i>N</i>,<i>O</i>-diacetyl-d-3-<i>epi</i>-daunosaminide and methyl <i>N</i>,<i>O</i>-diacetyl-d-ristosaminide
Asymmetric Syntheses of Methyl <i>N</i>,<i>O</i>‑Diacetyl‑d‑3-<i>epi</i>-daunosaminide and Methyl <i>N</i>,<i>O</i>‑Diacetyl‑d‑ristosaminide
Ab initio asymmetric syntheses of
methyl <i>N</i>,<i>O</i>-diacetyl-d-3-<i>epi</i>-daunosaminide
and methyl <i>N</i>,<i>O</i>-diacetyl-d-ristosaminide, employing diastereoselective epoxidation and dihydroxylation,
respectively, of alkyl (3<i>S</i>,α<i>R</i>,<i>Z</i>)-3-[<i>N</i>-benzyl-<i>N</i>-(α-methylbenzyl)amino]hex-4-enoates as the key steps, are
reported. The requisite substrates were readily prepared using the
conjugate additions of lithium (<i>R</i>)-<i>N</i>-benzyl-<i>N</i>-(α-methylbenzyl)amide to methyl
and <i>tert</i>-butyl (<i>E</i>)-hexa-2-en-4-ynoates
followed by diastereoselective alkyne reduction. <i>syn</i>-Dihydroxylation using OsO<sub>4</sub> proceeded under steric control
on the 4<i>Re</i>,5<i>Re</i> face of the olefin
to give the corresponding diol, which subsequently underwent lactonization.
Meanwhile, epoxidation using F<sub>3</sub>CCO<sub>3</sub>H in conjunction
with F<sub>3</sub>CCO<sub>2</sub>H proceeded on the opposite 4<i>Si</i>,5<i>Si</i> face of the olefin under hydrogen-bonding
control from the in situ formed ammonium ion. Treatment of the intermediate
epoxide with concd aq H<sub>2</sub>SO<sub>4</sub> promoted highly
regioselective ring-opening (distal to the in situ formed ammonium
moiety) to give the corresponding diol (completing overall the formal <i>anti</i>-dihydroxylation of the olefin), which then underwent
lactonization under the reaction conditions. Elaboration of these
diastereoisomeric lactones through hydrogenolysis, <i>N</i>-Boc protection, reduction, methanolysis, and acetate protection
gave methyl <i>N</i>,<i>O</i>-diacetyl-d-3-<i>epi</i>-daunosaminide and methyl <i>N</i>,<i>O</i>-diacetyl-d-ristosaminide
Asymmetric Syntheses of 3‑Deoxy-3-aminosphingoid Bases: Approaches Based on Parallel Kinetic Resolution and Double Asymmetric Induction
The
asymmetric syntheses of a range of <i>N</i>- and <i>O</i>-protected 3-deoxy-3-aminosphingoid bases have been achieved
using two complementary approaches. dl-Serine was converted
to a racemic <i>N</i>,<i>N</i>-dibenzyl-protected
γ-amino-α,β-unsaturated ester which was resolved
using a parallel kinetic resolution (PKR) strategy upon reaction with
a pseudoenantiomeric mixture of lithium (<i>R</i>)-<i>N</i>-benzyl-<i>N</i>-(α-methylbenzyl)amide
and lithium (<i>S</i>)-<i>N</i>-3,4-dimethoxybenzyl-<i>N</i>-(α-methylbenzyl)amide, giving the corresponding
enantio- and diastereoisomerically pure β,γ-diamino esters.
Alternatively, elaboration of l-serine gave the corresponding
enantiopure <i>N</i>,<i>N</i>-dibenzyl-protected
γ-amino-α,β-unsaturated ester, and doubly diastereoselective
conjugate addition of the antipodes of lithium <i>N</i>-benzyl-<i>N</i>-(α-methylbenzyl)amide was found to proceed under
the dominant stereocontrol of the lithium amide reagent in both cases,
thus augmenting the accessible range of β,γ-diamino esters.
Both of these protocols were expanded to include in situ oxidation
of the enolate formed upon conjugate addition, giving access to the
corresponding α-hydroxy-β,γ-diamino esters. Elaboration
of these β,γ-diamino and α-hydroxy-β,γ-diamino
esters gave the protected forms of the 3-deoxy-3-aminosphingoid base
targets
Asymmetric Syntheses of (−)-3-<i>epi</i>-Fagomine, (2<i>R</i>,3<i>S</i>,4<i>R</i>)‑Dihydroxypipecolic Acid, and Several Polyhydroxylated Homopipecolic Acids
A range of enantiopure polyhydroxylated
piperidines, including
(2<i>R</i>,3<i>S</i>,4<i>R</i>)-dihydroxypipecolic
acid, (−)-3-<i>epi</i>-fagomine, (2<i>S</i>,3<i>S</i>,4<i>R</i>)-dihydroxyhomopipecolic
acid, (2<i>S</i>,3<i>R</i>,4<i>R</i>)-dihydroxyhomopipecolic acid, and two trihydroxy-substituted homopipecolic
acids, have been prepared using diastereoselective olefinic oxidations
of a range of enantiopure tetrahydropyridines as the key step. The
requisite substrates were readily prepared from <i>tert</i>-butyl sorbate using our diastereoselective hydroamination or aminohydroxylation
protocols followed by ring-closing metathesis. After diastereoselective
olefinic oxidation of the resultant enantiopure tetrahydropyridines
and deprotection, enantiopure polyhydroxylated piperidines were isolated
as single diastereoisomers (>99:1 dr) in good overall yield
Asymmetric Syntheses of (+)-Preussin B, the C(2)-Epimer of (−)-Preussin B, and 3‑Deoxy-(+)-preussin B
Efficient de novo
asymmetric syntheses of (+)-preussin B, the C(2)-epimer
of (−)-preussin B, and 3-deoxy-(+)-preussin B have been developed,
using the diastereoselective conjugate addition of lithium (<i>S</i>)-<i>N</i>-benzyl-<i>N</i>-(α-methylbenzyl)amide
to <i>tert</i>-butyl 4-phenylbut-2-enoate and diastereoselective
reductive cyclizations of γ-amino ketones as the key steps to
set the stereochemistry. Conjugate addition followed by enolate protonation
generated the corresponding β-amino ester. Homologation using
the ester functionality as a synthetic handle gave the corresponding
γ-amino ketone. Hydrogenolytic N-debenzylation was accompanied
by diastereoselective reductive cyclization in situ; reductive N-methylation
then gave 3-deoxy-(+)-preussin B as the major diastereoisomeric product.
Meanwhile, the same conjugate addition but followed by enolate oxidation
with (+)-camphorsulfonyloxaziridine gave the corresponding <i>anti</i>-α-hydroxy-β-amino ester. α-Epimerization
by oxidation and diastereoselective reduction then gave access to
the corresponding <i>syn</i>-α-hydroxy-β-amino
ester. Homologation of both of these diastereoisomeric α-hydroxy-β-amino
esters gave the corresponding β-hydroxy-γ-amino ketones.
N-Debenzylation and concomitant diastereoselective reductive cyclization,
followed by reductive N-methylation, provided the C(2)-epimer of (−)-preussin
B and (+)-preussin B as the major diastereoisomeric products, respectively.
The overall yields (from phenylacetaldehyde) were 19% for 3-deoxy-(+)-preussin
B over seven steps, 8% for the C(2)-epimer of (−)-preussin
B over nine steps, and 7% for (+)-preussin B over eleven steps
Syntheses of Dihydroconduramines (±)-B-1, (±)-E-1, and (±)-F‑1 via Diastereoselective Epoxidation of N‑Protected 4‑Aminocyclohex-2-en-1-ols
Diastereoselective syntheses of dihydroconduramines
(±)-B-1,
(±)-E-1, and (±)-F-1 have been achieved from N-protected
4-aminocyclohex-2-en-1-ols via two complementary procedures for epoxidation
as the key step. Treatment of either <i>trans</i>- or <i>cis</i>-4-<i>N</i>-benzylaminocyclohex-2-en-1-ol with
Cl<sub>3</sub>CCO<sub>2</sub>H and then <i>m</i>-chloroperoxybenzoic
acid (<i>m</i>-CPBA) resulted in initial formation of the
corresponding ammonium species, followed by epoxidation on the face
syn to the ammonium moiety exclusively; chemoselective N-benzylation
then provided either (1<i>RS</i>,2<i>SR</i>,3<i>RS</i>,4<i>RS</i>)- or (1<i>RS</i>,2<i>RS</i>,3<i>SR</i>,4<i>SR</i>)-2,3-epoxy-4-<i>N</i>,<i>N</i>-dibenzylaminocyclohexan-1-ol, respectively.
Treatment of either <i>trans</i>- or <i>cis</i>-4-<i>N</i>,<i>N</i>-dibenzylaminocyclohex-2-en-1-ol
with <i>m</i>-CPBA resulted in initial formation of the
corresponding <i>N</i>-oxide, followed by epoxidation on
the face syn to the hydroxyl group exclusively; reduction then provided
either (1<i>RS</i>,2<i>RS</i>,3<i>SR</i>,4<i>RS</i>)- or an alternative route to (1<i>RS</i>,2<i>RS</i>,3<i>SR</i>,4<i>SR</i>)-2,3-epoxy-4-<i>N</i>,<i>N</i>-dibenzylaminocyclohexan-1-ol, respectively.
In all cases, S<sub>N</sub>2-type ring opening of these epoxides upon
treatment with aqueous H<sub>2</sub>SO<sub>4</sub> proceeded by nucleophilic
attack with inversion at C(2) preferentially, distal to the in situ
formed ammonium moiety. Hydrogenolytic N-deprotection then gave the
corresponding dihydroconduramines (±)-B-1, (±)-E-1, and
(±)-F-1
Probing Competitive and Co-operative Hydroxyl and Ammonium Hydrogen-Bonding Directed Epoxidations
The
diastereoselectivities and rates of epoxidation (upon treatment
with Cl<sub>3</sub>CCO<sub>2</sub>H then <i>m</i>-CPBA)
of a range of <i>cis</i>- and <i>trans</i>-4-aminocycloalk-2-en-1-ol
derivatives (containing five-, six-, and seven-membered rings) have
been investigated. In all cases where the two potential directing
groups can promote epoxidation on opposite faces of the ring scaffold,
evidence of competitive epoxidation pathways, promoted by hydrogen-bonding
to either the in situ formed ammonium moiety or the hydroxyl group,
was observed. In contrast to the relative directing group abilities
already established for the six-membered ring system (NHBn ≫
OH > NBn<sub>2</sub>), an <i>N</i>,<i>N</i>-dibenzylammonium
moiety appeared more proficient than a hydroxyl group at directing
the stereochemical course of the epoxidation reaction in a five- or
seven-membered system. In the former case, this was rationalized by
the drive to minimize torsional strain in the transition state being
coupled with assistance from hydrogen-bonding to the ammonium moiety.
In the latter case, this was ascribed to the steric bulk of the ammonium
moiety disfavoring conformations in which hydrogen-bonding to the
hydroxyl group results in direction of the epoxidation to the <i>syn</i> face. In cases where the two potential directing groups
can promote epoxidation on the same face of the ring scaffold, an
enhancement of epoxidation diastereoselectivity was not observed,
while introduction of a second, allylic heteroatom to the substrate
results in diminishment of the rate of epoxidation in all cases. Presumably,
reduction of the nucleophilicity of the olefin by the second, inductively
electron-withdrawing heteroatom is the dominant factor, and any assistance
to the epoxidation reaction by the potential to form hydrogen-bonds
to two directing groups rather than one is clearly unable to overwhelm
it