The complex photochemistry of 2,3-dibenzylidenesuccinates

Abstract

The photochemistry of diethyl E,E-2,3-(3,4,5-trimethoxybenzylidene)succinate (8) is solvent dependent. In both protic and aprotic solvents, there is a photoequilibrium established between 8 and its E,Z-isomer (9). In chloroform at high light intensity, very little 9 is formed and the main product is 1,4-dihydronaphthalene (10), formed via photoinduced intramolecular [1,3]-sigmatropic hydrogen shift within an intermediate 1,8a-dihydronaphthalene (11). In protic solvents, irradiation of either 8 or 9 ultimately gives primarily the cis-1,2-dihydronaphthalene product (13), along with smaller amounts of the trans isomer (14). By using deuterated solvents, it was shown that 13 and 14 are formed by solvent protonation (or deuteration) of the 1,8a-dihydronaphthalene intermediate (11 or 12). Key words: 2,3-dibenzylidenesuccinate, photocyclization, dihydronaphthalene, lignan.

Formal synthesis of (−)-podophyllotoxin through the photocyclization of an axially chiral 3,4-bisbenzylidene succinate amide ester – a flow photochemistry approach

“…13 (7). To an oven dried 250 mL three-necked round-bottom flask, equipped with a stirring bar and flushed with argon, 2.22 g (1.1 eq., 19.77 mmol) of t-BuOK was inserted, and was suspended in 65 mL of dry toluene. To the vigorously stirred suspension, a solution of 3.53 g (1 eq., 17.97 mmol) of 3,4,5-trimethoxybenzaldehyde and 5.0 g (1 eq., 17.97 mmol) of ester 6 in 50 mL of dry toluene was added dropwise.…”

“…Solvent protonation at C-2 was previously described as an alternative mechanism to a concerted [1,5]-sigmatropic hydrogen shift. 19 Indeed, we succeeded to obtain the desired product 12 when irradiation was performed in methanol. Further experiments confirmed that the use of an acidic additive (0.01 mM TFA) further accelerated the reaction, and helped to avoid excessive photolytic degradation, as proposed previously by Charlton.…”

“…In this paper, we explore the relatively less studied strategy to synthesize cyclolignans through the photocyclization of 3,4bisbenzylidenesuccinic acid amide esters. Although this kind of photocyclization was excessively studied for photochromic compounds belonging to the fulgide family (fulgides, fulgimides, and fulgenates), 19,20 there are only a few examples where it was employed in the synthesis of cyclolignans. [21][22][23] In 2004 Charlton et al used (−)-ephedrine as a chiral auxiliary to obtain a cyclic 3,4-bisbenzylidenesuccinic amide ester, which upon photocyclization resulted in enantiomerically pure (1S,2R)-trans-dihydronaphthalene (Scheme 1A).…”