Development of a New Spectral Tool for Identification of E and Z Isomers of Retinoids

Research Article

Austin J Anal Pharm Chem. 2017; 4(3): 1090.

Development of a New Spectral Tool for Identification of E and Z Isomers of Retinoids

Mohanraj SM1* and McMurry JE2

1PolyMicrospheres, Division of Vasmo Inc., Indianapolis, IN, USA

2Baker Laboratory, Department of Chemistry, Cornell University, Ithaca, NY, USA

*Corresponding author: Mohanraj SM [1], PolyMicrospheres, Division of Vasmo Inc., 4101 East 30th Street, Indianapolis, IN 46218, USA

Received: September 01, 2017; Accepted: September 27, 2017; Published: October 04, 2017

Abstract

A spectral study of a new series of fifteen retinoids based on the dimethyltetralin ring system, leading to the development of a new “Spectral Tool” for identification of E and Z isomers, is reported. Four types of retinoids (A-D), promising therapeutic agents for the treatment of epithelial cancer, were reported recently, in which various side-chain double bonds are rigidly held in specific conformations. Among the most active retinoids, the 9E isomers are 40-60 times more active than the corresponding 9Z isomers. In this study, a complete assignment of the 1H and 13C NMR signals of the retinoids to specific hydrogen and carbon atoms was achieved on the basis of chemical shifts, multiplicity, coupling constants, and assessment of substituent effects (i.e. shift changes upon alteration of the chemical structure), and comparisons of chemical shifts from compound to compound and of related compounds, and by proton noise-decoupled spectra and single frequency off-resonance decoupled spectra. The chemical shift differences between the 9E and the corresponding 9Z isomers are discussed, and interesting chemical shift patterns are observed. The distinct chemical shift patterns of the 9E and the 9Z retinoids in the 1H and 13C NMR could be used as a spectral tool to identify the E and Z isomers of other retinoids and vitamin A analogs.

Keywords: cis-and trans-retinoids; 1H and 13C NMR of E and Z isomers of retinoids

Introduction

The retinoids, a large class of polyunsaturated diterpenes structurally related to vitamin A, have aroused much interest because of their diverse biological properties [2] and versatile isomeric structures. In particular, the effect of vitamin A on epithelial tissues [2] has attracted much attention because vitamin A deficiency leads to hyperkeratosis of the skin and to metaplastic changes in the epithelia of gastrointestinal, respiratory, and urogenital tracts.

A number of synthetic retinoids have proven to be extremely effective in the treatment of various types of keratinization disorders [2]. Although a large number of retinoids have been synthesized, only limited work has been done on the NMR spectral study of retinoids and vitamin A derivatives [3,4].

The retinoic acid molecule is composed of three building units – a nonpolar cyclic end group, a polyene chain and a polar head group. Although it is relatively easy to probe structure-activity relationships involving the cyclic end group and polar head group, side-chain effects on biological activity are more difficult to study. The conformational flexibility of the side chain makes it possible for retinoids to adopt a large number of conformations, some of which are biologically active while others are inactive.

We recently reported [5] the development of four types of new retinoids containing dimethyltetralin end groups (A-D in Chart 1), in which various side-chain double bonds are rigidly held in specific conformations. Bicyclic type A analogs have the 5,6 and 7,8 double bonds of the retinoic acid locked into an s-cis conformation without disturbing the rest of the molecule, while tricyclic analogs of types B–D have additional constraints on double bond geometry imposed by introduction of a third ring.