Cyclohexanecarbonitriles: Assigning Configurations at Quaternary Centers from ¹³C NMR CN Chemical Shifts

13C NMR chemical shifts of the nitrile carbon in cyclohexanecarbonitriles directly correlate with the configuration of the quaternary, nitrile-bearing stereocenter. Comparing 13C NMR chemical shifts for over 200 cyclohexanecarbonitriles reveals that equatorially oriented nitriles resonate 3.3 ppm downfield, on average, from their axial counterparts. Pairs of axial/equatorial diastereomers varying only at the nitrile-bearing carbon consistently exhibit downfield shifts of δ 0.4−7.2 for the equatorial nitrile carbon, even in angularly substituted decalins and hydrindanes

Synthesis of (−)-α-Kainic Acid via TMSCl-Promoted Pd-Catalyzed Zinc-ene Cyclization of an Allyl Acetate

A highly practical synthesis of enantiopure (−)-α-kainic acid is accomplished in 37% overall yield, using 13 linear steps and a minimum of chromatographic separations via an unprecedented TMSCl-promoted palladium-catalyzed zinc-ene cyclization of an allyl acetate

Cyclic Nitriles: Stereodivergent Addition−Alkylation−Cyclization to <i>cis-</i> and <i>trans-</i>Abietanes

Diverse cyclic hydroxy nitriles are readily synthesized through sequential 1,2−1,4-Grignard addition−methylations to 3-oxo-1-cyclohexene-1-carbonitrile. Acid-catalyzed intramolecular cyclizations of the cyclic hydroxy nitriles reveal fundamental stereoselectivity trends in Friedel−Crafts cyclizations to cis- and trans-abietanes. In contrast to previous assumptions, comparative cationic cyclizations with electron-rich and electron-poor aromatic nucleophiles exhibit similar preferences for cyclization to cis-abietanes. Optimizing the cyclizations for trans-abietanes has identified ZrCl4 as an exceptional Lewis acid which, for cyclizations of iminolactones, favors trans-abietanes as the only observable diastereomer. The sequential oxonitrile addition−Friedel−Crafts cyclization strategy provides a rapid, stereodivergent synthesis of cis- or trans-abietanes, demonstrates the dramatic influence of ZrCl4 in promoting cationic cyclizations, and in contrast to previous assumptions suggests that the cyclization stereoselectivity is not correlated with the electronic nature of the aromatic nucleus

Cyclic Nitriles: Stereodivergent Addition−Alkylation−Cyclization to <i>cis-</i> and <i>trans-</i>Abietanes

Diverse cyclic hydroxy nitriles are readily synthesized through sequential 1,2−1,4-Grignard addition−methylations to 3-oxo-1-cyclohexene-1-carbonitrile. Acid-catalyzed intramolecular cyclizations of the cyclic hydroxy nitriles reveal fundamental stereoselectivity trends in Friedel−Crafts cyclizations to cis- and trans-abietanes. In contrast to previous assumptions, comparative cationic cyclizations with electron-rich and electron-poor aromatic nucleophiles exhibit similar preferences for cyclization to cis-abietanes. Optimizing the cyclizations for trans-abietanes has identified ZrCl4 as an exceptional Lewis acid which, for cyclizations of iminolactones, favors trans-abietanes as the only observable diastereomer. The sequential oxonitrile addition−Friedel−Crafts cyclization strategy provides a rapid, stereodivergent synthesis of cis- or trans-abietanes, demonstrates the dramatic influence of ZrCl4 in promoting cationic cyclizations, and in contrast to previous assumptions suggests that the cyclization stereoselectivity is not correlated with the electronic nature of the aromatic nucleus

Metalated Nitriles: Electrophile-Dependent Alkylations

Sequential carbonyl addition−conjugate addition to oxonitriles generates a C-magnesiated nitrile exhibiting electrophile-dependent alkylation stereoselectivities. Alkylations with alkyl halides, sulfonates, and ketones proceed with retention of stereochemistry, whereas aldehyde and acyl cyanide acylations proceed with inversion of stereochemistry. BuLi-initiated conversion of the C-magnesiated nitrile to the corresponding N-lithiated nitrile reverses the alkylation stereoselectivity, providing a facile route to diastereomeric nitriles that vary at a single, quaternary stereocenter

Oxonitriles: A Grignard Addition−Acylation Route to Enamides

Sequential addition of three different Grignard reagents and pivaloyl chloride to 3-oxo-1-cyclohexene-1-carbonitrile installs four new bonds to generate a diverse array of cyclic enamides. Remarkably, formation of the C-magnesiated nitrile intermediate is followed by preferential acylation by pivaloyl chloride rather than consumption by an in situ Grignard reagent. Rapid N-acylation of the C-magnesiated nitrile generates an acyl ketenimine that reacts readily with Grignard reagents or a trialkylzincate, effectively assembling highly substituted, cyclic enamides

Oxonitriles: A Grignard Addition−Acylation Route to Enamides

Sequential addition of three different Grignard reagents and pivaloyl chloride to 3-oxo-1-cyclohexene-1-carbonitrile installs four new bonds to generate a diverse array of cyclic enamides. Remarkably, formation of the C-magnesiated nitrile intermediate is followed by preferential acylation by pivaloyl chloride rather than consumption by an in situ Grignard reagent. Rapid N-acylation of the C-magnesiated nitrile generates an acyl ketenimine that reacts readily with Grignard reagents or a trialkylzincate, effectively assembling highly substituted, cyclic enamides

Cyclic Oxonitriles: Stereodivergent Grignard Addition−Alkylations

Sequential carbonyl addition−conjugate addition of Grignard reagents to cyclic 5−7-membered oxoalkenenitriles efficiently generates cyclic magnesiated nitriles. Alkylations of these magnesiated nitriles exhibit diastereoselectivities that depend intimately on the size of the carbocyclic ring:  5-membered oxonitriles generate magnesiated nitriles whose alkylations are controlled by steric constraints whereas 6- and 7-membered oxonitriles generate internally coordinated, C-magnesiated nitriles whose alkylations are controlled by stereoelectronic effects. Reversing the alkylation selectivity of 6-membered C-magnesiated nitriles is achieved by conversion to an N-metalated nitrile in which steric, rather than electronic, effects direct the electrophile trajectory. Collectively, the conjugate addition−alkylation generates highly substituted, cyclic 5−7-membered nitriles containing three new stereocenters with selective access to diastereomers at the quaternary nitrile-bearing carbon

Cyclic Oxonitriles: Stereodivergent Grignard Addition−Alkylations

Sequential carbonyl addition−conjugate addition of Grignard reagents to cyclic 5−7-membered oxoalkenenitriles efficiently generates cyclic magnesiated nitriles. Alkylations of these magnesiated nitriles exhibit diastereoselectivities that depend intimately on the size of the carbocyclic ring:  5-membered oxonitriles generate magnesiated nitriles whose alkylations are controlled by steric constraints whereas 6- and 7-membered oxonitriles generate internally coordinated, C-magnesiated nitriles whose alkylations are controlled by stereoelectronic effects. Reversing the alkylation selectivity of 6-membered C-magnesiated nitriles is achieved by conversion to an N-metalated nitrile in which steric, rather than electronic, effects direct the electrophile trajectory. Collectively, the conjugate addition−alkylation generates highly substituted, cyclic 5−7-membered nitriles containing three new stereocenters with selective access to diastereomers at the quaternary nitrile-bearing carbon

<i>C</i>-Metalated Nitriles: Electrophile-Dependent Alkylations and Acylations

Sequential carbonyl addition−conjugate addition of Grignard reagents to 3-oxocyclohex-1-ene-1-carbonitrile generates C-magnesiated nitriles whose alkylation stereoselectivities intimately depend on the nature of the electrophile. The alkylation of these C-magnesiated nitriles with alkyl halides, sulfonates, and unstrained ketones occurs with the retention of the CMg configuration, whereas aldehyde and acyl cyanide acylations proceed with inversion of the stereochemistry. Mechanistic probes indicate that the stereoselectivity is controlled by stereoelectronic effects for most electrophiles, except allylic, benzylic, and cyclopropyl halides where single-electron-transfer processes intervene. Screening numerous alkylations of C-magnesiated nitriles with a diverse range of electrophiles reveals the reaction scope and delineates the fundamental stereoelectronic effects responsible for the highly unusual electrophile-dependent alkylations