Cyclopentanols 1,2-disubstituted

The SiCaC reaction of 5-hexyn-l-al 23 gives the corresponding 2-(exo-silymethylene)-l-cyclopentanol 24 in high yield (Scheme 7.11) [20]. The reaction is accelerated by gem-disubstitution for example, the reaction of 3,3-gem-disubstituted 5-hexyn-l-al 23 (X = C(C02Et)2, C(CH20Me)2) is substantially faster and cleaner than the unsubstituted derivative (X=CH2), which is accompanied by a small amount of silylformylation product [21]. It should also be noted that the formation of l-siloxy-2-methylenecyclopentane is not observed, in sharp contrast with the nickel-catalyzed version of this reaction [20]. 

Both disubstituted alkynes (Chapter 3.3, this volume) and isolated terminal double bonds may be reduced by alkali metals in NH3, but isolated double bonds are usually stable to these conditions. However, 16,17-secopregnanes (10 equation 8) afford mixtures of cyclization products (11) and (12) in 61% to 80% yield with Na naphthalenide-THF, Na-NHs-THF, Na-THF or Li-NHs-THF. With Na-NHa-THF-r-butyl alcohol, a 91% yield of a 72 28 mixture of (11) (12) (R = Me) is obtained. This type of radical cyclization of alkenes and alkynes under dissolving metal reduction conditions to form cyclopentanols in the absence of added proton donors is a general reaction, and in other cases it competes with reduction of the carbonyl group. Under the conditions of these reactions which involve brief reaction times, neither competitive reduction of a terminal double bond nor an alkyne was observed. However, al-lenic aldehydes and ketones (13) with Li-NHs-r-butyl alcohol afford no reduction products in which the diene system survives. ... 

The di-Grignard method has also been applied to reactions of disubstituted amino [41,42], phenoxy, and phenylthio esters [42] to provide difunctional cycloalkanols examples include the conversion of esters 53 to cyclopentanols 54 [Eq. (20) 42]. Similarly, lactones can be converted to diols as in the formation of cyclopentanol 56 from lactone 55 [Eq. (21) 43]. 

Alkenylboronates activated with either an ester or a sulfone group have been engaged in [3+2] cycloadditions with methylenecyclopropanes to afford methylenecyclopen-tanes in good yields. The cycloaddition reactions were highly stereoselective and yielded exclusively the corresponding trans-disubstituted products. The carbon-boron bond of the cycloadducts can be further transformed by oxidation to give the corresponding cyclopentanols (Scheme 9.47) [lOOj. 

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