Cyclopentenes, synthesis

Thioxomalonates and a-thioxopropanoates have been prepared and subsequently trapped [220] with various dienes [221] (Table 3, entries 6 and 7). The resulting dihydro-2H-thiapyrans have been exposed to a strong base to effect a ring contraction providing a new synthesis of cyclopentenes [220]. Other adducts led to the formation of vinyl cyclopropanes, which are possible intermediates in cyclopentene synthesis. 

Very likely the ammonium fluorides are the proton sources and therefore the reason for incomplete conversions, since potassium fluoride in acetonitrile gives high yields in a very elegant [3 + 2]-annuIation process 87). It combines a Michael addition to a vinyl phosphonium salt with an intramolecular Wittig reaction and proceeds only in the presence of 18-crown-6 with satisfying yield. This cyclopentene synthesis has been executed in a repetitive manner to prepare linear triquinanes as illustrated in Scheme 6. Unfortunately, the sequence is non-stereoselective with regard to the ethoxycarbonyl functions. 

A detailed study from Takeda s group found that the exact mechanism for cyclopentene synthesis depended on the distal substituent of the unsaturated acyl silane (X in 75 below). With the thiophenyl group, after the Brook rearrangement, cyclization occurred via closure of the allyl anion 76 at the yend. For the vinyl silane compounds, however, Takeda s studies... 

Benjamin, L. K., Elizabeth, R. J. (2011). Palladium-catalyzed annulation reactions for diastereoselective cyclopentene synthesis. Organic Letters, 13, 4858-4861. 

A second common reaction of sulphonium ylides is Michael addition to ajS-unsaturated carbonyl compounds, giving cyclopropanes. This reaction forms the basis of a cyclopentene synthesis starting from sulphonium allylide (14). Other recent examples include the preparation of spiro[cyclopropane-l,4-A -pyrazolin]-5 -one derivatives (15) and cyclopropyl-ulose (16). Un-... 

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