Accordingly, it was stored as a solution in chloroform.Īctivated acyclic dienes 2a- c were explored and found to be quite reactive in Diels-Alder reactions with 1 ( Table 1). 7a,b Cyclobutenone ( 1) was found to be prone to polymerization when stored in the neat state, even at very low temperatures. We developed a modified version of the Sieja protocol in which a much milder Hunsdiecker-type decarboxylation-bromination was utilized, leading to gram-scale quantities of 1. Sieja 5 had actually described a preparation of cyclobutenone 1, following a very harsh Hunsdiecker-type decarboxylation-bromination of 3-oxo-cyclobutanecarboxylic acid 6 and subsequent elimination. 3) could well represent particularly useful platforms for further elaborations (vide infra).ĭCycloadduct 3h was isolated in 40% yield in the absence of ZnCl 2. Moreover, we could foresee that the products of such Diels-Alder reactions (cf. We anticipated that perhaps the ring strain of 1 might well serve to enhance its dienophilicity relative to corresponding cyclopentenones or cyclohexenones. Remarkably, although cycloadditions of substituted cyclobutenones have been reported, 4 we were unable to find a report of a Diels-Alder reaction of the parent cyclobutenone ( 1, Table 1). 3 While both of these dienophiles are known to undergo Lewis acid catalyzed Diels-Alder cycloaddition with less activated dienes, such applications require substrates which are stable to Lewis acids. However, under strictly thermal conditions, parent cyclenones such as cyclohexenone and cyclopentenone are relatively unreactive unless the diene contains strategically placed activating groups. 2Īccess to higher levels of complexity might well be facilitated if the dienophilic double bond in a Diels-Alder reaction is already housed in an existing ring. Indeed, the Diels-Alder reaction has proven to be a valuable resource in reaching many natural products of novel architecture. 1 The 6-membered rings created through Diels-Alder technology can be substructures of relatively simple targets or may be of value in building molecules of considerable complexity. The power of the Diels-Alder cycloaddition reaction in organic synthesis is widely appreciated.
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