Polysulfones as new organic catalysts : mechanism and development of new strategies for polyol selective semi-protection

Even unsubstituted butadiene adds to sulfur dioxide in the hetero-Diels-Alder mode more rapidly than in the chelotropic mode. The sultine can be observed in equilibrium with the diene and the sulfur dioxide only at low temperature and in the presence of CF3COOH. Crystals of the 4,5-dialkyl-sultine resulting from the SO2 addition to 1,2-dimethylidenecyclohexane have been obtained at -60 °C and analyzed by X-ray diffraction. Quantum chemistry calculations have shown that hyperconjugative interactions within the sulfinyl moiety are responsible for the anomeric effects observed in sultines that prefer pseudo-chair conformations with pseudo-axial S=O bonds. Mixing of sulfur dioxide and methylidenecyclopentane leads to the formation of alternating copolymer poly(methydilenecyclopentane sulfone) and isomerization of methylidenecyclopentane to 1-methylcyclopentene. Polysulfones are known to equilibrate with radicals upon heating (ceiling temperature of polysulfone formation). These sulfonyl radicals can abstract a hydrogen atom from methylidenecyclopentane (and from other 2-alkylalk-1-enes) to give an allyl radical type intermediate and sulfinic acid. The intermediate allyl radical abstracts an hydrogen atom from sulfinic acid and gives isomerized product (1-methylcyclopentene). Diphenyldisulfone or solid polysulfones are mild and efficient organic catalysts for selective cleavage of methylprenyl (2,3-dimethylbut-2-en-1-yl), prenyl (3-methylbut-2-en-1-yl), and methallyl (2-methylallyl) ethers. Our reaction conditions are compatible with the presence of other protecting groups such as acetals, acetates, and allyl, benzyl, and TBDMS ethers. Exposure of 2,3-dimethylbut-2-en-1-yl and 3-methylbut-2-en1-yl ethers to diphenyldisulfone or solid polysulfones led to the formation of 2,3-dimethylbuta-1,3-diene and isoprene, respectively. 2-Methylallyl ethers undergo isomerization to 2-methylpropenyl ethers, which are easily hydrolyzed into the corresponding free alcohols and isobutyraldehyde. We discovered that (2-methoxyprop-2-en-1-yl) silyl sulfinates are very effective agents for the silylation of alcohols and oximes. (2-methoxyprop-2-en-1-yl) silyl sulfinates allow easy, efficient and fast silylation of primary, secondy and tertiary alcohols. The diols are monosilylated at low temperature with high regioselectivity and high yields.