Umpolung of 1-alkoxy-3-acyloxy-1,3-dienes with sulfur dioxide and its application in polypropionate synthesis

Under appropriate conditions sulfur dioxide reacts with 1,3-dienes in the hetero-Diels-Alder fashion. In the case of 1-alkoxy-1,3-dienes the 6-alkoxy-3,6-dihydro-1,2-oxatiin-2-oxides (sultines) so-obtained can be ionized in the presence of Lewis or protic acid, thus realizing Umpolung of these electron-rich systems. The zwitterionic intermediates can be trapped as electrophiles with enoxysilanes (oxyallylation) to generate substituted silyl alkenylsulfinates. Desilylation of the latter under acidic conditions leads to intermediate allylsulfinic acids, which, after retro-ene desulfitation, produce 4-alkoxyhept-6-en-2-one core, a valuable fragment in polyketide natural product synthesis. This new SO2 reaction cascade, hetero-Diels-Alder addition/oxyallylation/retro-ene desulfitation (Vogel's cascade), was expanded to 1,3-dioxy-1,3-dienes. It was demonstrated that 1-alkoxy-3-(trialkylsilyl)oxy-1,3-dienes (Danishefsky's dienes) undergo ene reaction with sulfur dioxide due to migratory properties of trialkylsilyl substituent. The use of 1-alkoxy-3-acyloxy-2-methyl-1,3-pentadienes and enoxysilanes in Vogel's cascades resulted in an efficient synthesis of stereotriads containing keto and enol ester functions, suitable for further chain elongation. Good anti,syn and anti,anti selectivities were achieved using of (Z)- and (E)-enoxysilanes, respectively. The synthetic potential of the method was demonstrated by the short synthesis of 3,5-dihydroxycyclohexanone subunit of baconipyrones A and B as well as that of the hydroxydiketone subunit of baconipyrones C and D ((-)-(4S,6S)-4,6-dimethyl-5-hydroxynonan-3,7-dione). Further applications were shown by expeditious formal asymmetric total synthesis of rifamycin S and initiation of synthesis of C(27)-C(37) fragment of brasilinolides A and C. The method was extended to other π-nucleophiles, such as allylsilanes. A small library of differently substituted homoallylic ethers was synthesized to demonstrate utility of this new approach. Iterative Vogel's cascade employing newly obtained fragments provided long-chain polypropionate fragments such as (-)-(1Z,2S,3S,7S,8R,9R)-1-ethylidene-9-hydroxy-2,8-dimethyl-3,7-bis((1S,1'S)-phenylethoxy)-5-methylene-undec-1-yl benzoate in a very short and efficient way. Further generalization of π-nucleophiles led to the discovery of new ene reactions of SO2 with allylgermanes and allylboronates. A number of new chiral auxiliaries were prepared and tested in order to improve chemo- and diastereoselectivity in diene reactions with sulfur dioxide. The results demonstrated the superiority of inexpensive and readily available 1-arylethanols (and particularly 1-phenylethanol) over other types of chiral auxiliaries.