Cyclic Metalated Nitriles: Stereoselective Cyclizations to cis- and trans-Hydrindanes, Decalins, and Bicyclo[4.3.0]undecanes

Asymmetric Total Synthesis of Dankasterones A and B and Periconiastone A Through Radical Cyclization

We describe herein the assembly of the cis-decalin framework through radical cyclization initiated by metal-catalyzed hydrogen atom transfer (MHAT), further applied it in the asymmetric synthesis of dankasterones A and B and periconiastone A. Position-selective C-H oxygenation allowed for installation of the necessary functionality. A radical rearrangement was adopted to create 13(14→8)abeo-8-ergostane skeleton. Interconversion of dankasterone B and periconiastone A was realized through biomimetic intramolecular aldol and retro-aldol reactions. The MHAT-based approach, serves as a new dissection means, is complementary to the conventional ways to establish cis-decalin framework.

A Predictive Model Towards Site-Selective Metalations of Functionalized Heterocycles, Arenes, Olefins, and Alkanes using TMPZnCl⋅LiCl

The development of a predictive model towards site-selective deprotometalation reactions using TMPZnCl⋅LiCl is reported (TMP=2,2,6,6-tetramethylpiperidinyl). The pKa values of functionalized N-, S-, and O-heterocycles, arenes, alkenes, or alkanes were calculated and compared to the experimental deprotonation sites. Large overlap (>80 %) between the calculated and empirical deprotonation sites was observed, showing that thermodynamic factors strongly govern the metalation regioselectivity. In the case of olefins, calculated frozen state energies of the deprotonated substrates allowed a more accurate prediction. Additionally, various new N-heterocycles were analyzed and the metalation regioselectivities rationalized using the predictive model.

Enantioselective synthesis of α,α-disubstituted α-amino acids via direct catalytic asymmetric addition of acetonitrile to α-iminoesters

Chiral N-heterocyclic carbene complexes of [Ir(cod)(OMe)]₂ in combination with Barton's base enabled direct enantioselective addition of acetonitrile to α-iminoesters.

Stereochemical Course of Deprotonation-Acylation of N-Boc- and N-Carbamoyl-2-cyano-6-methylpiperidines

The stereochemical course of electrophilic substitution of α-nitrile metallocarbanions generated by deprotonation from N-Boc- and N-carbamoyl-2-cyano-6-methylpiperidines was investigated. Deprotonation in the presence of an electrophile taking advantage of the high acidity of α-nitrile protons allowed examination of the effects of a chelating group on the nitrogen atom, a countercation, and the reactivity of an electrophile on the steric course. Analyses of reactions using aroyl chlorides and methyl iodide revealed the following: (1) the substitution reactions basically proceed with retention of configuration, (2) the extent of an inversion product increases with decreasing chelating ability of the N-substituent and with increasing leaving ability (ionic character) of a countercation (Li, Na, K) of the anionic species, and (3) the use of a more reactive electrophile results in an increase of the retention product.

Metalated nitriles: SNi' cyclizations with a propargylic electrophile

A rare 5-exo-dig SNi' cyclization with magnesiated and lithiated nitriles affords a cis-fused hydrindane bearing an exocyclic allene. The cyclization of the dilithiated nitrile pits a stereoelectronic preference for a trans-hydrindane against a cyclization through a less strained transition structure to the corresponding cis-hydrindane. Computational modeling suggests that the dilithiated nitrile cyclizes to a cis-hydrindane because the preferred transition structure positions the lithium cation in a cone of electron density that bridges the nitrile-bearing carbon, an alkoxide, and an electron-rich alkyne functionality.

Regulation of diastereoselectivity in the carbocyclization of allenyl (S)-N-tert-butylsulfinimines through a three-component assembly

Allenyl sulfinimines can be stereoselectively cyclized with hexamethylditin under palladium catalysis conditions followed by a selection of additives for an activated transmetalation. Reactivity and diastereoselectivity for the cyclization strongly depend on the characteristics of additives. A highly diastereoselective synthesis of five-membered rings is achieved from the reaction of the corresponding allenyl (S)-N-tert-butylsulfinimies through the following sequence. After the distannylation of the allenyl group with hexamethylditin catalyzed by the Pd complex, stereochemical routes are additive dependent: addition of SnCl4 affords a cis ring exclusively, whereas a trans ring is formed predominantly by the introduction of B-bromocatecholborane. Extension of the methodology to the synthesis of six-membered cis rings is achieved by using B-bromocatecholborane. Stereochemical relationships of products were unambiguously deduced by X-ray crystallography.

Deslongchamps annulations with benzoquinone monoketals

The so-called Deslongchamps annulation of deprotonated γ,δ-unsaturated β-ketoesters 15 to 2-(alkoxycarbonyl)cyclohex-2-en-1-ones or similarly activated cyclohex-2-en-1-ones offers a versatile access to various kinds of decalindiones. The scope of Deslongchamps annulations was extended by establishing acceptor-substituted benzoquinone monoketals such as 13 as viable substrates. They gave octalindiones such as 35 with diastereoselectivities ≥ 95:5.

Fully substituted carbon centers by diastereoselective spirocyclization: stereoselective synthesis of (+)-lepadiformine C

Reductive lithiation of N-Boc alpha-amino nitriles generated alpha-amino alkyllithium reagents with unexpected selectivity. The intermediate radical prefers to align with the nitrogen lone pair, and this interaction leads to an A(1,3)-strain effect that biases the conformation of the radical. In cyclohexane rings with alpha-substituents the net effect is an inversion of configuration on reductive lithiation. In the presence of a tethered electrophile the alkyllithium cyclizes to produce a spiro compound, again with inversion of configuration. The overall result is retention of configuration in the cyclization reaction. The same overall selectivity is found with alpha-oxygen alkyllithium cyclizations, but in this case both steps proceed with retention. The difference can be explained by careful consideration of the intermediate geometries. The alpha-amino spirocyclization was utilized in a concise and stereoselective synthesis of lepadiformine C.

Cyclohexanecarbonitriles: assigning configurations at quaternary centers from (13)C NMR CN chemical shifts

(13)C NMR chemical shifts of the nitrile carbon in cyclohexanecarbonitriles directly correlate with the configuration of the quaternary, nitrile-bearing stereocenter. Comparing (13)C 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 delta 0.4-7.2 for the equatorial nitrile carbon, even in angularly substituted decalins and hydrindanes.