Ring transformations of five-membered heterocycles

Professor Nicolb Vivona and his colleagues from Palermo, Italy, have provided an overview of the present state of ring transformations of five-membered heterocycles which brings up-to-date a previous account by the same group published nearly 20 years ago in Volume 29 of these Advances. 

For reviews on ring transformation of five-membered heterocycles, see <93AHC(56)49> and CHEC-I <84CHEC-I(5)718>. 

Cusmano, G., Ring Transformations of Five-membered Heterocycles, 56, 49. 

Ring transformations of five-membered heterocycles and their fused derivatives 01EJ03405. 

The syntheses of five-membered heterocycles containing three oxygen or sulfur atoms in the 1,2,3-positions can be classified into three groups (i) one-component syntheses, in which a suitable acyclic molecule is cyclized by linkage of two heteroatoms (ii) two-component syntheses, in which the ring is built up either by a [3 -f 2] process or a [4 -1-1] process and (iii) synthesis by transformation of another heterocyclic ring. 

A number of very important synthetic methods are based on ring interconversions of six-membered heterocyclic systems. As with the transformations of five- into six-membered rings, examples are known of all of the possible atom fragments, but [5 + 1] and [4+2] processes are by far the most important. As described in Chapter 2.02, a number of [6+0] photoisomerizations of diazines have been discovered, but while these are of considerable mechanistic and theoretical importance they are of little preparative significance. 

A significant part of the examples of transition metal catalyzed formation of five membered heterocycles utilizes a carbon-heteroatom bond forming reaction as the concluding step. The palladium or copper promoted addition of amines or alcohols onto unsaturated bonds (acetylene, olefin, allene or allyl moieties) is a prime example. This chapter summarises all those catalytic transformations, where the five membered ring is formed in the intramolecular connection of a carbon atom and a heteroatom, except for annulation reactions, involving the formation of a carbon-heteroatom bond, which are discussed in Chapter 3.4. 

Preparations of cinnolines by expansion of five-membered heterocycle rings include the oxidation of A-aminooxindoles 232 (Scheme 132) <1988JHC847>, the treatment of isatogens 233 with ammonia (Scheme 133), and the base-catalyzed conversion of 1-aminodioxindoles 234 into cinnolin-3-ones 235 (Scheme 134) <1960JA4634>. Additional examples of the synthesis of cinnolines by transformation of another ring are available in CHEC-III . 

Reissert compounds, 9, I 24, 187 Ring closure of ortlio-substituted t-anilines, heterocycles by, 14, 211 Ring-opening of flve-membered heteroaromatic anions, 41, 41 Ring synthesis of heteroaromatic nitro compounds, 25, 113 Ring transformations and cyclizations on reaction of azines with bifunctional nucleophiles, 43, 301 of five-membered heterocycles, 56, 49... 

Many photochemical isomerizations of five-membered heterocycles having two or more heteroatoms including ring transformations have been... 

Aminofurans substituted with electron-withdrawing groups e.g. NO2) are known and 3-amino-2-methylfuran is a relatively stable amine which can be acylated and diazotized. 2-Amino-3-acetylfurans are converted into 3-cyano-2-methylpyrroles on treatment with aqueous ammonia. This transformation is a further illustration of the relative instability of the amino derivatives of five-membered ring heterocycles compared with anilines (Scheme 67) (781003821). 

The concept of five-membered ring heterocyclic synthesis by transformation of the initial adduct of the ADC compound and substrate is not limited to cyclization of substitution products. l,3,4-Oxadiazol-2-ones (30, Scheme 3) result from heating the initial DEAZD-dichlorocarbene adduct.72 Treatment of the Diels Alder adducts 96 with zinc in acetic acid gives pyrroles in good yield (Eq. 17).151 The reaction has been extended to the synthesis of dipyrroles from the appropriate Diels-Alder adduct (96, R = pyrrol-2-yl). 

The types of Si-containing five-membered heterocycles synthesized heretofore are shown in Figure 1. Only a few of them were known prior to 1980. Bora rings (l,m) deserve mention as the subject of a very broad and comprehensive study which revealed a series of peculiar transformations and structures. Rings with the internal dative bonds (e.g. k, bb (X = F)) add another dimension to ring... 

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