Epoxy hydroboration-oxidation

The hydroboration-oxidation studies of 1,4-epoxy-1,4-dihydronaphthalene (1) with various hydroborating agents such as borane-methyl sulfide (BMS), dicy-clohexylborane, disiamylborane, and 9-BBN have yielded interesting results [1]. The reaction of 1 with dicyclohexylborane or disiamylborane affords the exo alcohol 2 without epoxy ring opening (Eq. 32.1). On the other hand, 9-BBN hy-droboration of 1 (1 1 mole ratio) in THE at 25 °C, followed by oxidation affords a homoallylic alcohol 3 after opening of the epoxy ring (Eq. 32.2). BMS, however, affords a mixture of both the alcohols. 

On the other hand, in the derivatives of dicydohexylboranes and disiamylbo-ranes, the boron atom, that is surrounded by bulky substituents, finds it difficult to achieve coordination with the 7-oxa substituent. These derivatives on oxidation afford the epoxy-hydroxy compound 2. Consequently, the preparation of the ring opened compound 3 [1] via hydroboration-oxidation is superior to the literature method [9]. 

Synthesis of (A) started with the combination of 2,4,6-trimethylphenol and allyl bromide to give the or/Ao-allyl dienone. Acid-catalyzed rearrangement and oxidative hydroboration yielded the dienone with a propanol group in pore-position. Oxidation of the alcohol and bromolactonization (see p. 273) followed. Bromine and the lactone ring were trans in the product as expected (see p. 273). Treatment with aqueous potassium hydroxide gave the epoxy acid, which formed a crystalline salt with (R)-l-(a-naphthyDethylamine. This was recrystallized to constant rotation. 

Zinc-mediated Barbier-t) e addition of 238, followed by Luche s procedure obtained a mixture of the homoallylic alcohol diastereomers 239 and 240. Alcohol 239 was carried through benzoylation, deketalization, silylation, and ozonolysis, to produce C-branched y-lactone 241. Benzoylation of 240 followed by hydroboration, PCC oxidation, debenzoylation, and alkaline-promoted cyclization directly formed C3-branching 2-deoxyfuranose 242 [87] (O Scheme 63). A novel method for stereoselective synthesis of 4 -a-carbon-substituted nucleosides, through epoxidation of 4, 5 -unsaturated nucleosides and SnCU-promoted epoxy ring opening, was... 

Construction of the C-1 to C-9, fragment began with dienone 125, obtained from 2,4,6-trimethyl phenol and allyl bromide. Hydroboration of 125 followed by oxidation then gave a 72% yield of carboxylic acid 126. Bromolactonization of 126, which provided 127 (96%), set up the stereochemistry at what will later become the C-5 hydroxyl group. Epoxy keto acid 128 was subsequently obtained in 98% yield following alkaline hydrolysis. Corey reported that the resolution of 128 by recrystallization of the diastereomeric 1-a-naphthylethylamine salts was possible however, for the purposes of completion of the synthesis, racemic 128 was utilized. 

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