Abstract
The reaction of the mono-anion of the bromoallyl sulfone 1 with aldehydes 2 was examined with the aim of obtaining selectively substituted tetrahydrofurans. At -100oC syn- and anti- open chain adducts 3 and 4 were isolated together with a low yield of 4-methylene-2,3-disubstituted tetrahydrofuran 5. In the presence of HMPA or at higher temperature the reaction led to formation of 2,5-dihydrofurans 6. The stereochemical results are consistent with initial addition of 1 to the aldehyde involving Li ion chelation.
Highlights
We have shown 2,3 that 2-(bromomethyl)-3-phenylsulfonyl-1-propene 1 reacts with one molar equivalent of LDA to generate a lithiated α-allylsulfone carbanion that is stable at low temperature and undergoes regioselective and stereoselective additions via the α-carbon to Michael acceptors such as unsaturated esters, ketones, sulfones and nitro compounds, followed by cyclization to methylene-cyclopentenes
In view of general interest in formation of stereoselectively substituted tetrahydrofurans, such as the naturally occurring polyether antibiotics,4 we investigated the reaction of 1 with aldehydes as a potential entry into substituted tetrahydrofurans
We report here our findings, using the mono-lithio derivative of the bromoallyl sulfone 1 and of the hydroxyallylsulfone 8 in reactions with aldehydes 2
Summary
We have shown 2,3 that 2-(bromomethyl)-3-phenylsulfonyl-1-propene 1 reacts with one molar equivalent of LDA to generate a lithiated α-allylsulfone carbanion that is stable at low temperature and undergoes regioselective and stereoselective additions via the α-carbon to Michael acceptors such as unsaturated esters, ketones, sulfones and nitro compounds, followed by cyclization to methylene-cyclopentenes. The monolithio derivative generated was treated with p-nitrobenzaldehyde 2d at the same temperature and quenched after 30 min with HOAc to give a mixture of two open chain syn- and anti- isomers,5 3 and 4, as well as the trans- 2,3-disubstituted-4-methylenetetrahydrofuran 5 (see eq 2 and Table 1, entry 2).
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