Of chrom-3-enes

The aim of this review is to survey the advances in the chemistry of chrom-3-enes (2 -l-benzopyratis, chroin-/l3-eiies, a-chromenes)1 in the last decade. This subject was previously summarized by Wawzonek2 in 1951. A brief but clear treatment of chrom-3-ene chemistry can be found in Dean s book3 (1963), where a detailed account is given of all the natural products known at that time. A section of a recent book on the synthesis of natural compounds has been dedicated to chromenes.4... 

The nuclear magnetic resonance (NMR) spectrum of chrom-3-ene (1) has been measured13 and the sign of coupling constants between protons on the hetero ring obtained from a study of double quantum transitions.13 An inter-ring coupling (J4 8) has been detected.14-16 In... 

Much progress has been made in the last ten years in the synthesis of chrom-3-enes. Most of the activity has been stimulated by the discovery of new natural products and by the suggestions of possible biosynthetic pathways (see Section III). For results prior to 1963, the reader should consult Dean s book3 and Wawzonek s review.2... 

Hydroboration of chrom-3-ene, followed by H202 oxidation, gives predominantly chroman-4-ol, with little 3-isomer.244 The same happens for flavene, which gives a-flavan-4-ol,122 but when the aromatic ring bears electron-releasing substituents, boration occurs preferably at position 3.245... 

There has long been an interest in the chemistry of 2i/-chromenes or chrom-3-enes as a consequence of their widespread natural occurrence (B-63MI22400, B-73MI22400). The discovery that some methoxychromenes possess insecticidal properties stimulated activity in this area which led to the development of new and improved syntheses of chrom-3-enes. The reader is referred to two comprehensive surveys of chromene chemistry (75AHC(18)159, 77HC(31)11). 

The photochemical ring opening of chrom-3-enes (29) has been shown to produce o-quinoneallides (30). These have been trapped as the phenols (31) and (32) by their in situ reduction with lithium aluminum hydride (67JPC4045). The reverse process, thermal electrocyclic ring closure of the allide, is a facile reaction and hence any synthesis of an o -quinoneallide is effectively a synthesis of a chrom-3-ene. Many chromene syntheses are considered to proceed through the intermediacy of a quinoneallide and all of these are... 

A similar side-chain may be constructed from phenoxymagnesium halides and cinnamal-dehyde, when cyclization to the chromene occurs during acidic work-up. Unfortunately, this method tends to give mixtures of chrom-3-enes and chrom-2-enes, the latter arising by isomerization of the former. However, by careful choice of experimental conditions the former isomers can be obtained. The route has also been used in the synthesis of naph-thopyrans (71JCS(C)2546). 

The reaction of several amines with 7-methoxy-3-phenylbenzopyrylium perchlorate has been shown to yield the corresponding 2-substituted 3-phenylchromene (78JCS(P1)88), although the reaction of secondary amines with simple benzopyrylium salts gives mixtures of chrom-3-enes and chrom-2-enes (72JOC1069). 

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