Stereoselective Preparation of Vitamin D Precursors Using the Intramolecular Coupling of Alkynes and Cyclopropylcarbene−Chromium Complexes: A Formal Total Synthesis of (±)-Vitamin D₃

The intramolecular coupling of alkynes and cyclopropylcarbene−chromium complexes has been examined. Complexes that feature a stereogenic center at the propargylic position of the alkyne−carbene tether are the focus of this paper. The reaction produces a cyclopentadienone intermediate fused to an oxygen heterocycle, which is reduced to the corresponding cyclopentenone under the reaction conditions (100 °C in 1% aqueous toluene). The preexisting stereogenic center has a powerful influence on the reduction of the cyclopentadienone ring, and predominantly a single diastereomer is produced in the reaction. Reductive cleavage of the heterocyclic ring with retention of stereochemistry affords compounds featuring a stereocenter on the five-membered ring and on a side chain. Use of the above reaction processes for the synthesis of the vitamin D precursor de-ABC-cholestan-14-one is also discussed

Stereoselective Preparation of Vitamin D Precursors Using the Intramolecular Coupling of Alkynes and Cyclopropylcarbene−Chromium Complexes: A Formal Total Synthesis of (±)-Vitamin D₃

The intramolecular coupling of alkynes and cyclopropylcarbene−chromium complexes has been examined. Complexes that feature a stereogenic center at the propargylic position of the alkyne−carbene tether are the focus of this paper. The reaction produces a cyclopentadienone intermediate fused to an oxygen heterocycle, which is reduced to the corresponding cyclopentenone under the reaction conditions (100 °C in 1% aqueous toluene). The preexisting stereogenic center has a powerful influence on the reduction of the cyclopentadienone ring, and predominantly a single diastereomer is produced in the reaction. Reductive cleavage of the heterocyclic ring with retention of stereochemistry affords compounds featuring a stereocenter on the five-membered ring and on a side chain. Use of the above reaction processes for the synthesis of the vitamin D precursor de-ABC-cholestan-14-one is also discussed

Relative Asymmetric Induction in the Intramolecular Reaction between Alkynes and Cyclopropylcarbene−Chromium Complexes: Stereocontrolled Synthesis of Five-Membered Rings Fused to Oxygen Heterocycles

Synthesis of cyclopentenone derivatives fused to oxygen heterocycles by means of the intramolecular coupling of alkynes and cyclopropylcarbene−chromium complexes has been examined for a variety of cases in which the tethering chain features a stereogenic center. In some cases, this process proceeds with a very high degree of stereoselectivity; however, the extent and direction of relative asymmetric induction is very dependent upon the position of the chiral atom within the tethering chain and the length of the tethering chain. In the best case, featuring a two-carbon tether and a stereogenic center at the homopropargylic position (complex 1N), the heterocyclic ring was produced with 97:3 selectivity for the cis heterocycle (3N). In the worst case, featuring a three-carbon tether and a stereogenic center at the homopropargylic position (complexes 1F and 1I), no stereoselectivity was observed. Improvement in stereoselectivity was noticed when terminal alkynes were replaced by silylated alkynes and when proton sources were eliminated from the reaction

Relative Asymmetric Induction in the Intramolecular Reaction between Alkynes and Cyclopropylcarbene−Chromium Complexes: Stereocontrolled Synthesis of Five-Membered Rings Fused to Oxygen Heterocycles

Synthesis of cyclopentenone derivatives fused to oxygen heterocycles by means of the intramolecular coupling of alkynes and cyclopropylcarbene−chromium complexes has been examined for a variety of cases in which the tethering chain features a stereogenic center. In some cases, this process proceeds with a very high degree of stereoselectivity; however, the extent and direction of relative asymmetric induction is very dependent upon the position of the chiral atom within the tethering chain and the length of the tethering chain. In the best case, featuring a two-carbon tether and a stereogenic center at the homopropargylic position (complex 1N), the heterocyclic ring was produced with 97:3 selectivity for the cis heterocycle (3N). In the worst case, featuring a three-carbon tether and a stereogenic center at the homopropargylic position (complexes 1F and 1I), no stereoselectivity was observed. Improvement in stereoselectivity was noticed when terminal alkynes were replaced by silylated alkynes and when proton sources were eliminated from the reaction

Relative Asymmetric Induction in the Intramolecular Reaction between Alkynes and Cyclopropylcarbene−Chromium Complexes: Stereocontrolled Synthesis of Five-Membered Rings Fused to Oxygen Heterocycles

Synthesis of cyclopentenone derivatives fused to oxygen heterocycles by means of the intramolecular coupling of alkynes and cyclopropylcarbene−chromium complexes has been examined for a variety of cases in which the tethering chain features a stereogenic center. In some cases, this process proceeds with a very high degree of stereoselectivity; however, the extent and direction of relative asymmetric induction is very dependent upon the position of the chiral atom within the tethering chain and the length of the tethering chain. In the best case, featuring a two-carbon tether and a stereogenic center at the homopropargylic position (complex 1N), the heterocyclic ring was produced with 97:3 selectivity for the cis heterocycle (3N). In the worst case, featuring a three-carbon tether and a stereogenic center at the homopropargylic position (complexes 1F and 1I), no stereoselectivity was observed. Improvement in stereoselectivity was noticed when terminal alkynes were replaced by silylated alkynes and when proton sources were eliminated from the reaction

Kras^G12D mutation led to mucinous metaplasia in pancreatic ducts.

<p>H&E staining of a typical lesion in mouse pancreas (A) and its connection to normal ductal epithelium in a nearby serial section (B) with alcian blue staining to denote mucin-producing cells. Arrowhead, junction between nonmucinous and mucinous cells; arrows, junction between cuboidal and columnar cells. (C) Mucinous lesions were all strongly positive for claudin-18 (brown) with highest concentrations along lateral cell borders. Size bars, 50 µm.</p

Kras^G12D mutation led to adenoma formation in lungs.

<p>A, B. PAS and hematoxylin staining of lung adenomas typical of CK19^CreERT; LSL-Kras^G12D mice. Size bars, 50 µm.</p

Cre-mediated recombination of the LSL-Kras^G12D allele in different tissues.

<p>A. DNA was extracted from the indicated tissues of three different mice and subjected to 35 cycles of PCR that would detect both the wildtype Kras allele (271 bp) and the recombined LSL-Kras^G12D allele (310 bp) but not the unrecombined LSL-Kras^G12D allele. Tissues examined: 1) tail, 2) small intestine, 3) liver, 4) pancreas, 5) kidney, 6) stomach, and 7) colon. Strong recombined bands were detected for small intestine, pancreas and colon and weaker bands for stomach, kidney and liver, while recombination was never detected in tail DNA. B. The amount of recombination of an unaffected tissue, the small intestine, was compared to the amount in an oral papilloma in which all or nearly all of the epithelium should have a recombined Kras allele. For three different mice, DNA was prepared from total oral tissue (lanes 1–3), isolated papilloma tissue (lanes 4–6) and total small intestine tissue (lanes 7–9) and subjected to 30 cycles of PCR. Little recombination can be detected in total oral tissue while the recombination is abundant in the isolated papilloma (upper band in each lane). The amount of recombination in the small intestine was intermediate between these two populations.</p

Kras^G12D mutation led to foveolar hyperplasia in the gastric fundus.

<p>A. PAS (pink) showed extensive mucin production deep into affected glands while surrounding normal glands only had staining at the tops of glands. B. Immunolabeling for phosphohistone H3 as an M phase marker. The proliferative zone in affected glands was shifted toward the base of the glands. C. Immunolabeling for TFF2 (brown) showed positive cells near base of affected fundic glands while normal glands had expression in the normal mucous neck region. Arrows, staining in affected glands; arrowheads, staining in normal glands. Size bars, 50 µm.</p

Morbidity analysis: epithelial expression of Kras^G12D led to weight loss.

<p>Mice were monitored weekly for weight and other indications of overall health. Number maintaining at least 80% of highest body weight is plotted as a function of time for CK19^CreERT; LSL-Kras^G12D (solid line) and for littermates also injected with tamoxifen (dashed line). An additional 3 control mice were also injected with tamoxifen but were only followed for 3–5 months as controls for earlier timepoints and did not lose weight during that time (data not shown).</p