The Copper(I) Catalysed [2 + 2] Intramolecular Photoannulation of Carbohydrate Derivatives

Abstract

Carbohydrate derivatives 1a, 1b, 3a and 3b have been subjected to copper(I) catalysed photoannulation to produce tetracyclic products 2a, 2b, 4a and 4b respectively as single enantiomers. The same reaction carried out on substrates 5a and 5b leads stereoselectively to the spiro products 6a and 6b respectively. Photoannulation of substrate 7 gave an approximate 1:1 mixture of spiro products 8 and 9. A mechanistic rationalisation of these results is proposed.Synthetic chemists have used carbohydrates as polyhydroxylated chiral starting materials for many years. 1 When the chosen target molecule in these ventures is cyclic, methods for the annulation of the carbohydrate starting material are required, and there are many varied strategies to achieve this objective. 2 One potentially useful method for carbohydrate annulation involves a [2+2] photoannulation. To the best of our knowledge there are three applications of intramolecular enone-olefin [2+2] photoannulation in a carbohydrate example, 3 in which stereoselectivity was not always observed. There are also two examples of intermolecular enone-olefin [2+2] photoannulation on sugar derivatives, one of which occurs in low yield with poor stereoselectivity. 4 We now report the stereoselective formation of several cyclobutane carbohydrate derivatives, in good yield, via the copper(I) catalysed [2+2] photoannulation of the corresponding unactivated 1,6-dienes.The formation of coordination complexes between copper(I) and olefins is well known. 5, 6 Applications of these complexes in catalysed photoannulation reactions has been studied by Salomon for many years. 7 In particular copper(I) catalysed [2+2] cycloaddition has been especially successful. Coordination of the copper(I) to both olefins produces a complex which absorbs the light and cyclizes. The product does not coordinate, and so is released from the copper, which then coordinates another molecule of diene to complete the cycle.Continuing our interest in new methods for carbohydrate annulation 8 we have investigated the application of the copper(I) triflate catalysed [2+2] photoannulation to this problem. We were particularly interested to see if the promising results of Salomon and Ghosh, 9 on the stereocontrol of these reactions, could be applied in carbohydrate examples to produce single enantiomerically pure diastereoisomers.Downloaded by: UC Santa Barbara. Copyrighted material.

[2+2] Photochemical Cycloaddition in Organic Synthesis

“…On the other hand, an alkyl substituent at the allylic position makes the reaction to proceed through the exo ‐bidentate complex producing preferentially exo ‐substituted bicyclo[3.2.0]heptane. The influence of allylic hydroxyl group during [2+2] photocycloaddition has nicely been demonstrated by stereoselective annulations of bicyclo[3.2.0]heptanes 148 and 150 on pyrano sugar derivatives 147 and 149 respectively (Scheme ) …”

Diasterocontrol in the intramolecular meta-photocycloaddition of arenes and olefines

“…Irradiation of the diastereoisomer (RS,SR)-16 produced the same result as for (SS,RR)-11 leading to the polycyclic product 18. This structural assignment was based on 1 H-NMR, 13 C-NMR, 1 H-COSY and 1 H-NOESY experiments and finally confirmed by an X-ray crystal structure of the p-bromobenzoylester derivative of 18 (Fig. 1).…”

“…Flash column chromatography was performed on Sorbsil C-60 silica gel (Crosfield Chemicals), 40-60 lm. NMR spectra were recorded on a Bruker DPX 300 ( 1 H, 300.13 MHz; 13 (3S,5S),(3R,5R)-5-Phenyl-1,6-heptadien-3-ol (10) 3-Phenyl-4-pentenal 9 (0.683 g, 4.26 mmol) in THF (10 mL) was added dropwise to vinyl magnesium bromine (1.0 M in THF; 15 mL) while cooling in ice. The reaction progress was followed by TLC (petroleum ether : ether/10 : 1).…”

“…As part of a programme of research on the copper(I) triflate catalysed [2p + 2p] cycloaddition reactions we carried out the reaction sequences described in Scheme 1. 13 Cinnamyl alcohol 7 was reacted with ethyl vinyl ether and mercuric acetate to produce allyl vinyl ether 8 14 (Scheme 3).Claisen rearrangement of 8 gave the aldehyde 9 15 which was then reacted with vinyl magnesium bromide to furnish the alcohol 10 as a 1 : 1 mixture of diastereoisomers. Irradiation of 10 as a 1 : 1 mixture of diastereoisomers not surprisingly gave a photo-cyclisation products also as a mixture of non-aromatic isomers.…”

Formation of a Four‐Membered Ring

“…A similar transformation of an enolether was used as key step in the synthesis of the sequiterpene a-cedrene [31]. Esters [32], carbamates [33], or carbohydrate derivatives [34] possessing two alkene double bonds have all been successfully transformed, and the resulting products applied to organic synthesis.…”