3- Carene, boranes from

Among chiral dialkylboranes, diisopinocampheylborane (8) is the most important and best-studied asymmetric hydroborating agent. It is obtained in both enantiomeric forms from naturally occurring a-pinene. Several procedures for its synthesis have been developed (151—153). The most convenient one, providing product of essentially 100% ee, involves the hydroboration of a-pinene with borane—dimethyl sulfide in tetrahydrofuran (154). Other chiral dialkylboranes derived from terpenes, eg, 2- and 3-carene (155), limonene (156), and longifolene (157,158), can also be prepared by controlled hydroboration. A more tedious approach to chiral dialkylboranes is based on the resolution of racemates. /n j -2,5-Dimethylborolane, which shows excellent enantioselectivity in the hydroboration of all principal classes of prochiral alkenes except 1,1-disubstituted terminal double bonds, has been... 

Both carenes 40 and 41 are readily hydroborated by borane dimethyl sulfide complex to give the diisocaranylboranes (Icr.BH) which can be isolated in pure form. Treatment w ith methanol gives the S-methoxy derivatives, which readily react with allylic carbanions to give the corresponding allyldiisocaranylboranes 42-45. These compounds have been used for asymmetric allyl additions to carbonyl compounds (Sections D. 1.3.3.3. and D.2.5.2.). The reagents 44 and 45, derived from 2-carene39, are more enantioselective than those from 3-carene(1. 

The systematic studies conducted by Midland and coworkers [8] have shown that among the various B-alkyl-9-BBN obtained from (+)-a-pinene, (-)-(3-pi-nene, (-)- camphene, and (+) 3-carene (Chart 26.2), B-3-pinanyl-9-BBN (Alpine-Borane) shows remarkable enantioselectivity as illustrated in Table 26.1 [8]. 

Many other allylborating agents (12-21) utilizing chiral auxiliaries derived from carenes, camphor, tartaric acid, stein, etc. are also known (Figure 2) (2i-33). Recently Villieras described an ester-containing chiral allylborating agent which could be used directly for the synthesis of a-methylene-y-butyrolactones (32). In this review, we have restricted our discussions to the synthesis of lactones via homoallylic alcohols derived from B-allyldiisopinocampheyl-borane, 1. 

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