Carbenoid anion behavior of dilithio derivatives of thioacetal alcohols. Stereochemistry and mechanism of ring closures by oxyanion-facilitated carbon-hydrogen bond insertion

Abstract

Like other lithio derivatives of thioacetals bearing a second anionic site, dilithio derivatives of 3,3-, 5 5 , and 6,6-bis(phenylthio) alcohols decompose at or below ambient temperature to yield products ascribable to carbenoid intermediates. The 5,5-derivatives decompose far faster than the other types, and they yield mainly 2-(phenylthio)cyclopentanols and none of the unsaturated alcohols and glycols, arising from 1,2-hydrogen transfer and carbenoid dimerization, respectively, which are the major products from the other two types of reactants. The five-membered ring formation observed in the 5,5-systems results from insertion of a carbenoid carbon atom into the activated carbinol CH bond of the oxyanion, and a stereochemical study reveals that this process is not concerted. The evidence suggests that this ring closure is a result of displacement of a thiophenoxide ion from the lithiothioacetal by a hydride ion which is transferred from the carbinyl carbon atom in a transition state involving coordination of the lithium atom of the carbenoid with the oxyanion.Recently, we proposed the following principle which has led A. J. Am. Chem. Soc. 1973, 95,4100. Brown, H. C.; Krishnamurthy, S. J. J. Am. Chem. Soc. 1972,94,7159. (12) Consistent with literature precedents, 23 and 21 were produced in a ratio of 2.6:l; for example, see: Dauben, W. G.; Fonkcn, G. J.; Noyce, D. S. J. Am. Chem. Soc. 1956, 78, 2519. (13) Mura, A. J., Jr.; Majetich, P. A.; G r i m , P. A.; Cohen, T. Tetrahedron Lett. 1975, 4431. (14) The major product of treatment of the cis-cyclohexanol mixture 21 with excess HCl and 1.5 quiv of thiophenol for 20 h was the octahydrochromene resulting from intramolecular attack of the hydroxyl group on the cation produced by protonation of the vinyl sulfide group, but, as expected, (see, for example, Scheme HI) further treatment of this material led to the monocyclic thioacetal in good yield. An analogous product was formed from 23 along with a much larger amount of 24 after a 5-h treatment with HCI in neat thiophenol (40 quiv). (15) (a) Crandall, J. K.; Magha, H. S. J. Org. Chem. 1982,47,5368. (b) Crandall, J. K.; Magha, H. S.; Henderson, M. A,; Widner, R. K.; Tharp, G.

Single Enantiomer Epoxides by Bromomandelation of Prochiral Alkenes

“…It is particularly noteworthy that these displacements, to give 12 and 13, led to the enantiomerically-pure cis derivatives 8 13 and 14, 5,14 not directly available by other means. Indeed, the simple alcohol 8 13 had not previously been described in enantiomerically-pure form. X-ray analysis of the crystalline 9 allowed assignment of the absolute configuration of 7a.…”

“…The secondary bromide of 7a (Scheme ) participated more efficiently in nucleophilic displacement after oxidation to the corresponding ketone 11 . It is particularly noteworthy that these displacements, to give 12 and 13 , led to the enantiomerically pure cis derivatives 8 and 14 , , not directly available by other means. Indeed, the simple alcohol 8 13 had not previously been described in an enantiomerically pure form.…”

Reactions of Allylmagnesium Reagents with Carbonyl Compounds and Compounds with C═N Double Bonds: Their Diastereoselectivities Generally Cannot Be Analyzed Using the Felkin–Anh and Chelation-Control Models

“…Sometimes, stereochemical information was not provided. Stereochemical assignments can also be misleading, even in cases of well-studied reactions such as reductions of 2-substituted cyclohexanones, where different papers might suggest different stereochemical outcomes (as illustrated by comparing the following two papers discussing a different type of reaction than discussed here , ). It would have been helpful for researchers to provide that information, or in other cases, when an allylmagnesium compound was used, to provide insight into what happened (if anything) with other reagents.…”

Highly selective electrochemical fluorination of dithioacetal derivatives bearing electron-withdrawing substituents at the position α to the sulfur atom using poly(HF) salts

“…At first, anodic fluorination of ethyl α,α-bis(phenylthio)acetate ( 1b ) [ 30 – 31 ] was carried out at platinum plate electrodes in an undivided cell using various solvents in the presence of Et 3 N·3HF as supporting salt and fluorine source. A constant current was passed until the starting material 1b was completely consumed (monitored by TLC).…”