I. All carbon inverse-electron-demand Diels-Alder reactions : exploration of the chemistry of an electron deficient diene. II. Synthesis of some 6,15-disubstituted 2,11-dithia[3.3] metacyclophanes

Abstract

Dienes which bear electron withdrawing groups at the 1- and 3- positions are formal electronic complements of dienes such as Danishefsky's diene. Surprisingly, the inverse-electron-demand Diels-Alder (IEDDA) chemistry of these types of dienes has received little attention in the literature to date. The synthesis of (2E)-3-(1'-oxo-2'-cyclohexen-2'-yl)-l-phenyl-2-propen-l-one (102), a novel electron deficient diene substituted in the 1- and 3-positions with electron withdrawing groups, is discussed. This diene and, to a greater extent, its direct synthetic precursor acetal (101) are stable when stored under nitrogen at -20 °C, allowing them to be isolated and stored for later use. Like other electron deficient dienes prepared previously in our laboratory, the diene acetal (101) was observed to participate in a normal-electron-demand Diels-Alder reaction, while the deprotected diene (102) underwent IEDDA reactions with a variety of electron rich dienophiles, including ethyl vinyl ether, 2-(trimethylsilyl)oxy-3,4-dihydrofuran (127), and 1,1-diethoxyethene (131). -- The use of enamines as dienophiles led to the formation of dihydronaphthalenones. A domino IEDDA-elimination-dehydrogenation reaction is postulated to account for the formation of these products. The dehydrogenation was proposed to be a result of hydrogen transfer to excess dienophile or other unsaturated species present during the reaction. -- The chemistry of two additional dienes, 7-benzoyl-6-ethoxy- (133) and 7-benzoyi-6-hydroxy-3,4,4a,5-tetrahydro-1(2H-naphthalenone (167), derived from the IEDDA adduct of diene 102 with 131, was also examined. The ethoxy substituted diene (133) was observed to oxidize in the presence of a range of potential dienophiles, and the hydroxy substituted diene (167) was unreactive as a diene in an IEDDA cycloaddition under the conditions investigated. -- As part of a cooperative effort within our group to study the solution state conformational preferences of substituted 2,ll-dithia[3.3]metacyclophanes, 6,15-dibromo- (198) and 6,15-diiodo-2,11-dithia[3.3]metacyclophane (199), as well as 2,11-dithia[3.3]metacyclophane (192) have been synthesized. The 6,15-difluoro- (196) and 6,15-dichloro-2,ll-dithia[3.3]metacyclophanes (197) were also targeted for synthesis, however the Na₂S/Al₂O₃ coupling technique used was unsuccessful for these compounds. The attempt to prepare the dichloro- substituted cyclophane (197) resulted in the isolation of the trimer, 6,15,24-trichloro-2,11,20-trithia[3.3.3]metacyclophane (211), in modest yield.