Epoxide with sodium cyanoborohydride

Oxirane cleavage. Isobe and co-workers effected the stereoselective reduction of 1 to 2 by sequential treatment of the epoxide with sodium cyanoborohydride in anhydrous HMPT and diborane in THF. The authors suggest that this reaction involves internal hydride transfer in the hypothetical intermediate 3. Reduction in DME or THF results in products with the opposite configuration at C7. 

We invoke 7c-stacking of the alkene with a phenyl moiety on the silicon protecting group (since this high degree of selectivity was only observed for TBDPS and not with TBS), in the precursor to explain this remarkable selectivity (Figure 1). Lewis acid induced reduction of the epoxide with sodium cyanoborohydride led regioselectively to the 1,3-diol (11) the hydride attacks the more substituted position via an S 2 mechanism. ... 

In contrast to the reductive cleavage of 1-methylcyclohexene epoxide with LiAlH4 or, better, with LiEtjBH to produce 1-methylcyclohexanol, reduction of the epoxide with sodium cyanoborohydride in the presence of boron trifluoride etherate furnishes cE-2-methylcyclohexanol. In this case, complexation of the epoxide oxygen with the Lewis acid BF3 now directs hydride addition to the more substituted carbon, which can better sustain the induced partial positive charge. 

When triptolide 1 was reacted with nucleophillic reagents HBr, CH3CH2CH2SH, HOAc, MeOH, ammonium thiocyanide or 4-methoxybenzenesulfonyl chloride, the same C12-C13 epoxide reacted to form compounds 69-73 [70-72], Refluxing 1 for 96 hrs in phosphate buffer solution at pH 4 generated triptriolide 10 in 43% yield [72], In contrast, when 1 was stirred with sodium cyanoborohydride followed by dropwise addition of neat boron trifluoride diethyl etherate at room temperature for 16 hrs, the epoxide ring at C-7-C-8 opened and the compound 74 was obtained. Subsequently, reaction of compound 74 with HC1 at room... 

The reductive ring opening of 330a with sodium cyanoborohydride/titanium tetrachloride in acetonitrile occurs with no ester reduction whatsoever to provide 421 in 83% yield. Subsequent conversion to the tosylate followed by reduction with lithium borohydride/lithium triethylborohydride affords in 61% yield the crystalline diol 422. Lithium aluminum hydride or sodium borohydride reduction of the tosylate of 421 fails to produce clean reductions to 422. Epoxide ring closure of 422 is achieved with two equivalents of sodium hydroxide in methanol to fiimish in 93% yield (2 S, 3i )-2-benzyloxy-3,4-epoxybutan-l-ol (423) [140] (Scheme 94). 

Acid catalysed opening of the epoxide 47 in the presence of benzylamine afforded 6-ben lamino-l,4,6-trideoxy-l,4-imino-D-mannitol as product, whereas in the absence of ben lamine the rearranged fert-butyl ether 48 was the major isolate. The same paper also describes the preparation of the 7-membered ring product 49 by reacting bic clic hemiaminal 50 (prepared in six steps from benzyl-4-azido-4-deoxy-2,3-0-isopro idene-a-D-mannopyranoside) with sodium cyanoborohydride followed by hydrolysis. ... 

The preparation of the CD building block was continued as follows. After reductive opening of the epoxide 91 with sodium cyanoborohydride, the resulting diol was converted into the bis-acetyl ester 92. Selective hydrolysis of the less sterically hindered ester was followed by conversion of the unprotected alcohol into the thiocarbonate. After deoxygenation via a Barton-McCombie reaction, the ensuing product 93 is set up for deprotection and oxidation to a key chiral CD intermediate for vitamin D synthesis. 

A double nucleophilic attack by benzylamine on a symmetric bis-epoxide, prepared from D-mannitol, was used in a recent synthesis of 250 [564], In another recent report, 5-keto-D-glucose was reacted with diphenylmethylamine and sodium cyanoborohydride in a key step in the formation of 250 [565]. 

Squalene 2,3-epoxide has been isolated from the green alga Caulerpa prolifera. Oxidation of squalene with t-butyl hydroperoxide in the presence of Mo02(acac)2 and di-isopropyl (+)-tartrate gave the 2,3-epoxide (31%) with an induced asymmetry of about 14% in favour of the (35)-isomer. The ability of oxidosqualene cyclases to accept unnatural precursors has been further extended by the observation that lanosterol cyclase from rabbit liver converts the synthetic epoxide (1) into the jS-onocerin derivative (2). An authentic sample of (2) was prepared by sodium cyanoborohydride reduction of /3-onoceradione... 

Regioselective ring opening of epoxides to the less-substituted alcohol, by hydride capture at the better potential carbenium ion centre, is achieved with a sodium cyanoborohydride-boron trifluoride combination anti ring opening is favoured. 

The reduction of alkyl hahdes has been important in many syntheses. Sodium cyanoborohydride in HMPA will reduce alkyl iodides, bromides, and tosylates selectively in the presence of ester, amide, nitro, chloro, cyano, alkene, epoxide, and aldehyde groups [118]. Tri-n-butyltin hydride will replace chloro, bromo, or iodo groups with hydrogen via a free radical chain reaction initiated by thermal decomposition of AIBN [119]. Other functionality such as ketones, esters, amides, ethers, and alcohols survive unchanged. The less toxic tris(trimethylsilyl) silane can be used similarly [120]. 

---