Regio- and stereospecific synthesis of a new benzoconduritol-C derivative

Chemistry of Unsymmetrical C1-Substituted Oxabenzonorbornadienes

Oxabenzonorbornadiene (OBD) is a useful synthetic intermediate, which can be readily activated by transition metal complexes with great face selectivity due to its dual-faced nature and intrinsic angle strain on the alkene. To date, the understanding of transition-metal catalyzed reactions of OBD itself has burgeoned; however, this has not been the case for unsymmetrical OBDs. Throughout the development of these reactions, the nature of C1-substituent has proven to have a profound effect on both the reactivity and selectivity of the outcome of the reaction. Upon substitution, different modes of reactivity arise, contributing to the possibility of multiple stereo-, regio-, and in extreme cases, constitutional isomers, which can provide unique means of constructing a variety of synthetically useful cyclic frameworks. To maximize selectivity, an understanding of bridgehead substituent effects is crucial. To that end, this review outlines hitherto reported examples of bridgehead substituent effects on the chemistry of unsymmetrical C1-substituted OBDs.

Recent Advances in Transition Metal-Catalyzed Reactions of Oxabenzonorbornadiene

BACKGROUND

Oxabenzonorbornadiene (OBD) is a useful synthetic intermediate capable of undergoing multiple types of transformations due to three key structural features: a free alkene, a bridged oxygen atom, and a highly strained ring system. Most notably, ring-opening reactions of OBD using transition metal catalysts and nucleophiles produce multiple stereocenters in a single step. The resulting dihydronaphthalene framework is found in many natural products, which have been shown to be biologically active.

OBJECTIVE

This review will provide an overview of transition metal-catalyzed reactions from the past couple of years including cobalt, copper, iridium, nickel, palladium and rhodium- catalyzed reactions. In addition, the recent derivatization of OBD to cyclopropanated oxabenzonorbornadiene and its reactivity will be discussed.

CONCLUSION

It can be seen from the review, that the work done on this topic has employed the use of many different transition metal catalysts, with many different nucleophiles, to perform various transformations on the OBD molecule. Additionally, depending on the catalyst and ligand used, the stereo and regioselectivity of the product can be controlled, with proposed mechanisms to support the understanding of such reactions. The use of palladium has also generated a cyclopropanated OBD, with reactivity similar to that of OBD. An additional reactive site exists at the distal cyclopropane carbon, giving rise to three types of ring-opened products.

Rhodium-catalysed Nucleophilic Ring Opening Reaction of 1- and 3-ethoxy-5,8-epoxy-5,8-dihydroisoquinolines

2-Ethoxy-3- and 5-chloropyridines were obtained from 2,3- and 2,5-dichloropyridine. Reaction of 2,3- and 2,5-dichlo-ropyridines with tBuLi in the presence of furan gave 1- and 3-ethoxy-5,8-epoxy-5,8-dihydroisoquinolines. The rhodium-catalysed ring-opening reaction of 1- and 3-ethoxy-5,8-epoxy-5,8-dihydroisoquinolines with 2-bromophenol furnished the isomer 7-(2-bromophenoxy)-1-ethoxy-7,8-dihydroisoquinolin-8-ol, 6-(2-bromophenoxy)-1-ethoxy-5,6-dihydroisoquinolin-5-ol and 7-(2-bromophenoxy)-3-ethoxy-7,8-dihydroisoquinolin-8-ol, 6-(2-bromophenoxy)-3-ethoxy-5,6-dihydroisoquinolin-5-ol respectively.

A Novel and Stereospecific Synthesis of Aminocyclitol: N-Tosyldihydroconduramine E2

A stereospecific synthesis of N-tosyldihydroconduramine E2, a new aminocyclitol, has been synthesised starting from cyclohexa-1,3-diene. The photooxygenation of cyclohexa-1,3-diene afforded the bicyclic endoperoxide. Reduction of the endoperoxide with thiourea and then reaction of the bis-carbamate with p-TsNCO followed by a palladium-catalysed ionisation/cyclisation reaction, gave a vinyl oxazolidin-2-one. Oxidation of the double bond in the oxazolidin-2-one with KMnO4 followed by acetylation, gave the oxazolidinone-diacetates. Hydrolysis of the oxazolidinone ring and removal of the acetate groups gave the desired aminocyclitol, N-tosyldihydroconduramine E2.

Synthesis and Characterisation of 2,3,4a,6,8a-penta-acetoxy decahydronaphthalene from 1,2,3,4-tetrahydronaphthalen-2-ol

2,3,4a,6,8a-Penta-acetoxy decahydronaphthalene has been synthesised starting from 1,2,3,4-tetrahydronaphthalen-2-ol. The Birch reduction of 1,2,3,4-tetrahydronaphthalen-2-ol gave a high yield of 1,2,3,4,5,8-hexahydronaphthalen-2-ol which was acetylated. The OsO4 oxidation of the 1,2,3,4,5,8-hexahydronaphthalen-2-yl acetate followed by further acetylation gave the pentaacetates.

Synthesis and Characterisation of 2,3a,5,6,7a-pentaacetoxy-octahydro-1H-indene from indan-2-ol

A synthesis of 2,3a,5,6,7a-pentaacetoxyoctahydro-1 H-indene has been achieved starting from indan-2-ol. The Birch reduction of indan-2-ol gave 4,7-dihydro-2-indanol in high yield which when followed by acetylation, resulted in the corresponding acetates. The OsO4 oxidation of the 2-acetoxy-4,7-dihydroindane followed by acetylation furnished the desired pentaacetates in high yield.