Chiral synthesis via organoboranes. 20. Conversion of boronic esters of essentially 100% optical purity to B-alkyl-9-borabicyclo[3.3.1]nonanes of very high optical purity. Synthesis of optically active homologated esters, nitriles, and ketones

Abstract

ChemInform Abstract Asymmetric hydroboration of prochiral olefins such as (I) and subsequent removal of the chiral auxiliary yields the optically active borinane (VI), which is converted into a mixture of the borabicyclo(4.2.1)nonane (VIIIa) and -(3.3.1)nonane (IXa). By this route, seven mixtures of isomeric, optically active borabicyclononanes such as (VIII) and (IX) are prepared; on thermal isomerization the more stable (4.2.1)isomers can be isolated without loss of optical purity. The mixtures react readily with α-halogenated esters, ketones and nitriles such as (X) to give the products (XI) with high optical purities. Since both (+)-and (-)-α-pinene are readily available as chiral auxiliaries, a simple procedure for the synthesis of optically active compounds is given. The use of sterically more hindered alkyl groups in the borabicyclononanes gives poor results.

Aromatic aldols and 1,5-diketones as optimized fragrance photocages

“…This approach is useful for the liberation of simple terpene hydrocarbons such as β-pinene (1) as was demonstrated for the photocage 2. This cage was prepared by the t-butoxide-mediated metal-free coupling protocol developed by Brown et al 16 This cage exclusively cleaves to the fragrance 1 and acetophenone when irradiated with UV light (Scheme 1). 17 A quantum yield of >90% was determined by valerophenone actinometry.…”

Hydroboration. 94. Rates of Hydroboration of 2-Organylapopinenes with 9-Borabicyclo[3.3.1]nonane, ProvidingB-(2-Organylapoisopinocampheyl)-9-borabicyclo[3.3.1]nonanes, Potentially Valuable for the Asymmetric Reduction of Prochiral Ketones

“…The second stage is the intramolecular cyclic hydroboration, which favors formation of the five-membered boracyclane over the six-membered ring (eq 5). The less stable 1,4-regioisomer is readily isomerized to the 1,5-isomer at 65 °C …”

Catalytic Asymmetric Hydroboration: Recent Advances and Applications in Carbon−Carbon Bond‐Forming Reactions

“…[120] Reduction of the BϪO bond is effected by treatment with LiAlH 4 (which generates LiRBH 3 ), followed by TMSCl, to afford RBH 2 . Thus, after enantioselective hydroboration, O'Donnell trapped the initially formed catecholate with pinacol, and then transformed the BPin substituent into a 9-BBN group using a procedure reported by H. C. Brown.…”