Grignard reagent-promoted selective ring expansion and alkylation of formyl borneol and isoborneol: a new route to highly substituted cyclopentanes

Selective Ring Expansion Alkylation of Formyl[2.2.1]bicyclic Carbinols with C-Nucleophiles: A Unique Route to Cyclopentane Derivatives

“…As mentioned above, the reactions of camphor-based formyl-[2.2.1]bicyclic carbinol (6) with various Grignard reagents have been previously described. 9 Now, the new finding is that carbinol 6 also reacted with lithium reagents and gave the same products as those provided by the reactions with corresponding Grignard reagents (Table 1). Thus, individual reactions of 6 with both types of C-nucleophile donors showed the same chemoselectivity and regio-and stereochemical (>99% de) behaviors.…”

“…The mechanism of reaction of 6 with Grignard reagents has been previously illustrated. 9 Thus, it is expected that the mechanism of the reaction of 6 with alkyl/aryl lithium compounds is similar to that of the reaction of 6 with Grignard reagents.…”

“…However, the yields of products 15, 21, and 22 were significantly improved after the reaction time was increased (entries 6, 7, 14, 15, 17, and 18). Although the reactions with allylMgCl, n-BuMgCl, vinylMgBr, and CH 3 CCMgBr (entries,8,9,11,and 12) were individually carried out in 15 and 30 min, the yields of the corresponding products given in both time periods were almost the same. Interestingly, the reaction with phenyl ethynyl magnesium bromide (entry 13) also afforded the expected product in good yield.…”

“…However, the reaction of formyl borneol (6) with either 1 or 2.5 equiv of various Grignard reagents provided alkyl[3.2.1]bicyclic diols instead of ketones. 9 It should be emphasized that 5 underwent ring expansion only, 8 whereas 6 did consecutively undergo a ring expansion-alkylation reaction when both of them were individually treated with 1 equiv of the same reagent. These interesting results impelled us to further study the reactions of other formyl[2.2.1]bicyclic carbinol analogues with some C-nucleophiles.…”

Reduction of Alkynyl α‐Hydroxy Esters: Stereoselective α‐Ketol Rearrangement

“…[1][2][3] Under these conditions, both isomers are in equilibrium, and the more stable α-hydroxy carbonyl compound is formed. This transformation, called α-ketol or acyloin rearrangement, has been applied to the synthesis of many polycyclic compounds.…”