Intramolecular Hetero-Diels-Alder Reaction of Vinylallenes and Imines: Synthesis of 9-Methyl-1,2,3,4,5,6,7,8-octahydroacridine

Abstract

The intramolecular hetero-Diels-Alder reaction of vinylallenes and imines has been carried out. Depending on the substituents on the allene and the length of the tether linking diene and heterodienophile, the reaction proceeds thermally or with Lewis acid activation. The transformation of one of the adducts into a 9-substituted octahydroacridine is reported.In the last few years, we have been interested in the synthetic applications of the hetero-Diels-Alder reaction using vinylallenes as dienes. We found that semicyclic vinylallenes 1 can react with aldehydes in the presence of Lewis acids, giving the corresponding cycloadducts with a hexahydrochromone skeleton (Equation 1). 2 The reaction proceeds with complete regio-and facialselectivity, with good endo/exo selectivity and in fair to good yields. Equation 1When these semicyclic vinylallenes were reacted with Nbenzylimines, the adducts with an octahydroquinoline skeleton were obtained as a single isomer with cis-stereochemistry in each case (Equation 1). 3 The yields were similar to those obtained in the reaction with aldehydes.In these reactions, some limitations were found, the most important one being the need for a substituent on the allene carbon bonded to the vinyl moiety (R 1 in Equation 1). This substituent could act as an activating group facilitating the reaction and also controlling the regiochemistry. It is important to note that for a similar reaction to take place using nonallenic dienes, a strong electron-releasing group, such as alkoxy or silyloxy, needs to be present, whereas with vinylallenes it suffices to have a simple alkyl group as substituent.The intramolecular Diels-Alder reaction is generally more favorable than the intermolecular process, mainly due to entropic reasons, 4 and although only a few examples of intramolecular Diels-Alder reaction of dienes with a carbonyl group as dienophile have been reported, 5 we decided to test the vinylallene moiety in this reaction with aldehydes as heterodienophiles. The required compounds containing the semicyclic vinylallene and the aldehyde linked by a chain of two or three carbon atoms were prepared, and it was found that the reaction took place yielding only one isomer (trans) in each case 6 due to the restrictions imposed by the length of the tether and the rigidity of the allene 7 (Equation 2). The compounds with the shorter chains were more reactive, as expected, 7 and in fact some of the reactions were spontaneous at room temperature (R = alkyl, n = 1, Equation 2). What was more interesting was the finding that the reaction took place even without any alkyl substituent on the allene carbon bonded to the vinyl group (R = H, n = 1, Equation 2), although in this case heating or Lewis acid activation was needed. With the longer chain compounds (n = 2) only the substituted vinylallenes reacted (R = alkyl) with heating or Lewis acid catalysis. Equation 2Since in the intermolecular version of the reaction the imines reacted with higher selectivity than the aldehydes, we decided to prepare the syst...