Sodium bis(methoxyethoxy)aluminium hydride

Abstract

(A) Koide et al. reported the use of NaBH4 or Red-Alreg; to achieve the reduction of propargylic alcohols to allylic alcohols. They demonstrated that Red-Alreg; gave only E alkenes whereas NaBH 4 gave E/Z mixtures. Superior yields were obtained with Red-Al reg; compared to NaBH4 and fewer equivalents were required. It has also been shown that the metalated intermediates can be intercepted by an electrophile, thus allowing derivatization of the alkene in a single operation. (figure presented) (B) Igawa and Tomooka reported the regioselective reduction of a bisalkyne to an E alkene. Following the preferential order of reactivity TPS > Ph ∼ TBDPS > TIPS ∼ TMS ≫ Alk, Red-Al gave regioselectivities from 72:28 to 97:3 and 79100% yields. Other hydride reagents were reported to be less selective than Red-Alreg; (C) Shibasaki et al. recently reported the use of Red-Alreg; in the reduction of α,β-epoxy peroxy esters to aldehydes in high yields. In this interesting transformation, the reduction of such an ester could be achieved without epoxide opening. (D) En route to the synthesis of dicyclopenta[a,e] pentalenes, Cao et al. successfully utilized Red Alreg; in the reduction of a tetrayne to the corresponding bisallenebisalkyne under high dilution conditions. Several reducing reagents including LiAlH4 proved unsuccessful prior to achieving satisfactory results with Red-Alreg;. The reduction presumably occurs by formation of an aluminum complex of the propargylic alcohol and subsequent delivery of hydride in an S N2′fashion. (E) Ghosh et al.9 used Red-Al reg; successfully in combination with a copper salt to reduce α,β-unsaturated ketones to saturated ketones. It is believed the reaction proceeds via the formation of a copper hydride species. In the absence of copper salts only the carbonyl is reduced and the olefin remains intact. (F) Voight et al.11 used Red-Alreg; for the efficient preparation of chiral diamines via selective reduction of N-Boc-protected amino acid derived secondary amides. The reactions proceeded with minimal overreduction or cyclic urea formation. Furthermore, no epimerization was observed during the reduction. The methodology was also extended to selective reduction of di- and tripeptides in excellent yields. (G) The use of Red-Al for the selective opening of 2,3-epoxy alcohols has been one of the earliest reported uses. 12 While studying the Payne rearrangement Yamazaki et al. reported the use of Red-Alreg; for the regioselective opening of an epoxy alcohol to furnish a 1,3-diol. (H) Corey et al. reported the transformation shown; the regioselectivity of reduction could be controlled by the addition of a base. Other hydride reagents, such as NaBH4, LiBH4, L-selectride, and 9-BBN-H were unsuccessful. The first step gave a single stereoisomer and both steps gave quantitative yields. (I) Towards the synthesis of serotonin antagonists, Cid et al. used Red-Alreg; for stereoselective carbonyl reduction in the molecule; the cis-alcohols were prepared with excellent diastereoselectivities and yields. (J) In a short synthesis towards prostaglandin E1 Sih et al. reduced the alkoxy substituent from the ring with 5060% yields. © Georg Thieme Verlag Stuttgart - New York.