Chrom-3-enes synthesis

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... 

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... 

The present volume encompasses a wide range of heterocyclic chemistry. Syntheses of heterocycles from thioureas are reviewed by T. S. Criffin, T. S. Woods, and 1). L. Klayman, while S. W. Schneller describes the chemistry of benzothiins and their derivatives (thiochromans, thiochromones, and thio-chromanones). Developments in chrom-3-ene chemistry are reviewed by L. Merlini. F. D. Popp contributes a chapter on the isatins. A discussion of theoretical aspects of the tautomerism of pyrimidines, by J. S. Kwiatkowski and B. Pullman, follows up a corresponding earlier contribution (Vol. 13) on tautomeric purines. In the final chapter P. and D. Cagniant describe the natural occurrence and synthesis of the benzofurans. 

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). 

Another example illustrates the utility of ethyl acetoacetate in the synthesis of chrom-2-enes. Reaction of the /3-keto ester with the benzyl halide (173) affords the hydroxyphenylpropanone (174) which forms the chromene with acetic anhydride (Scheme 27) (42JA435). 

When heated with polyphosphoric acid, 2-ethoxychromans lose ethanol to give the chrom-2-ene (67CB1296). The method constitutes a synthesis of chrom-2-enes from coumarins, since the chromans are obtained in two steps from the latter. 

Bcnzo[/)]furan and derivatives, recent advances in chemistry of, Part 1, occurrence and synthesis, 18, 337 Benzo[c]furans, 26, 135 recent advances in the chemistry of, and related compounds, 73, 1 Benzofuroxans, 10, 1 29, 251 2//-l-Benzopyrans (chrom-3-enes), 18, 159 Bcnzofc] pyrylium salts syntheses, reactions, and physical properties, 50, 157 1,2-and 2,1-Benzothiazines and related compounds, 28, 73... 

Another interesting example of sulfone photochemistry pertains to diyne 92, which was investigated as an early approach for the synthesis of the highly unstable nine-membered ene-diyne ring of neocarzinostatin Chrom A 1 by Wender et al. [56]. The benzophenone-sensitized desulfonation of 92 resulted in the formation of the desired, but highly unstable, compound 93 in 9-15% yield (Scheme 2.22). 

G. Sartori, G. Casiraghi, L. Bolzoni, and G. Casnati, General synthesis of 2//-bcnzo fc]pyrans (chrom-3-enes) from metal phenoxides and ,p-unsaturated carbonyl compounds,/. Org. Chem., 44, 803-805 (1979). 

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