Wavelength Control in the Photochemical Addition of Tricyclic [g]‐Fused 1‐Thiocoumarins or Coumarins to Alkenes Yielding Mono‐ and Bis([2 + 2]‐Cycloadducts)

Abstract

On long-wavelength (>395 nm) irradiation in the presence same alkene furnishes mixtures of all bis(photocyc1oad-of 2,3-dimethylbut-2-ene the benzobis(thiin0nes) 6 and 7 af-ducts), of which some were isolated by chromatography and ford 4 : 1 mixtures of cis-and trans-fused monocycloadducts.analyzed by 'H-NMR spectroscopy. -Mixed bis([2 + 21-phoSimilarly, irradiation of thiinobenzopyrandione 8 affords 50 % tocycloadducts) of such molecules become easily accessible, of a 4 : l mixture of cis-and trans-fused monocycloadducts at as illustrated by the irradiation of benzobispyranone 9, first the thiinone moiety and in addition further 50% of the cis-(>350 nm) in the presence of 2,3-dimethylbut-2-ene and fused monocycloadduct at the pyranone ring. Irradiation of then (>300 nm) in the presence of isobutene. 6-8 at shorter wavelength (>340 nm) in the presence of the We recently communicated results on the photochemical behaviour of 2H-benzothiopyran-2-one (1-thiocoumarin, 1) both in solution ['] and in the solid state [*]. In reactions in the presence of alkenes we found ['] that irradiation of 1 in benzene containing an excess of 2,3-dimethylbut-2-ene affords a 4:l mixture of the corresponding cis-and trunsfused [2 + 21-cycloadducts 2 and 3, respectively. Under similar conditions irradiation of coumarin (4) yields only the cis-fused cycloadduct 5t31 (Scheme 1). This reflects the higher flexibility of the thiinone ring in excited 1 as compared to the pyranone ring in 4 which is due to the differences in bond lengths (C-s-C > C-0-C). 3We now investigated the behavior of the extended thiocoumarins 6-8 as well as that of the pyranocoumarin 9 (Scheme 2) in light-induced [2 + 21-cycloadditions. In this paper we discuss results on the wavelength dependence of these reactions which allows us to synthesize mono-and/or bis(cyc1oadducts) selectively. For the synthesis of the novel tricycles 7 and 8 we started from appropriately substituted benzenedicarbaldehydes (Scheme 3) using a similar sequence involving double condensation reaction with triethylphosphonoacetate, saponification, and cyclization of the bis(cinnamic acid) as published for the preparation of 6L4,'1. Thus, 2,5-bis(dimethylethy1thio)benzene-1,4-dicarbaldehyde (10) -prepared from 2,5-dibromobenzene-1,4-dicarbaldehyde (11)f6] -and 4-(dimethylethylthio)-6-methoxybenzene-1,3-dicarbaldehyde (1 2) -prepared from 4-bromo-6-methoxybenzene-1,3-dicarbaldehyde (13)"l in two steps via phenol 14 -were converted to 7 and 8 via diesters 15/16 and diacids 17/18, respectively.Irradiation of 6-8 with light of h > 395 nm -wavelengths where 1-thiocoumarin (1) (h,,, = 340 nm) shows almost no absorption -saturated (5 . M) in acetonitrile in the presence of an excess of 2,3-dimethylbut-2-ene

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

“…The Margaretha group also investigated the site selectivity in several thiocoumarin-containing bichromophoric compounds. ,,, A few examples 446 – 448 are shown in Figure , with the more reactive bonds toward a [2 + 2] photocycloaddition with 2,3-dimethyl-2-butene being marked.…”