Ring-enlargement reactions

Another possibility is to use the high reactivity of cyclic dialkyne systems toward electrophiles in order to add the additional carbon atom to the ring. When treated with dichloro-carbene at low temperature, cyclic dialkynes give mainly cyclic diynones [Eq. (18)]. In the same fashion, cyclononyn-3-one (91), the smallest conjugated cycloalkynone yet, has recently been made [Eq. (19)] [35]. 

---

By Ring-Enlargement Reactions from Thiapyrylium Salts... 

A photolytic reduction of the 5-chloro-substituted tetrazolo[l,5- ]pyridine derivative 60 was observed by Dias et al. <1996JHC1035> (Scheme 17). These authors found that the photolysis of the starting compound 60 when carried out with unfiltered light followed an unusual pathway instead of a ring-enlargement reaction experienced with use of Pyrex filter in many cases, a photolytic reduction takes place in 44% yield, and the chlorine substituent is replaced by... 

Finally, three ring-enlargement reactions should be referenced in this section. Gerecke et al. reported <1994H(39)693> that treatment of the 5-chloromethyl-substituted triazolo[4,3-c]quinazoline compound 141 with sodium hydroxide results in the formation of a seven-membered diazepine ring 142 in good yield. 

When the terminal alkynes 96 are treated with the trimethylsilylalkyne 97 in the presence of HfCl4 as a Lewis acid, the silylated vinylallenes 98 are produced in acceptable yields. In an intramolecular variant of this process, 100 was obtained from the diyne 99 [32]. Vinylallenes, incorporated into a cyclic framework and hence of restricted conformational mobility, are of interest for photochemical studies [33] and are among the photoproducts in ring-enlargement reactions of polycyclic allenes [34]. 

Though the fragmentation is one of the basic reactions of radical ions, this destructive reaction pathway seems to be synthetically useless at first sight. Nevertheless, the electron-transfer-induced bond cleavage can be specific and, for this reason, synthetically useful (e.g., for ring enlargement reactions). 

Scheme 12 Ring enlargement reaction by cyclobutane ring cleavage.

The formation of macrocycles through ring-enlargement reactions via bicyclic tetrahedral intermediates—i.e., the cyclols of general type 89 or 92 (the ionized forms are shown), which are mostly unstable and can be... 

Much research into the ring-enlargement reactions of thietanes has been done by Takamizawa et A variety of reactions were carried out... 

Treatment of 4,4-dimethyl-2-phenyl-l,3-thiazole-5(4//)-thione with ethyl diazoacetate gives, among other products, ethyl 1,3-thiazine carboxylate (179) (99). The formation of 179 has been rationalized by an acid-catalyzed addition of ethyl diazoacetate to the thiocarbonyl ylide 177 to first give intermediate 178, which undergoes a subsequent ring enlargement reaction via a Tiffeneau-Demjanov rearrangement. 

Diazabicyclo[3.2.0]hept-2-en-7-ones 2 gave diazepinone derivatives 3 in a base-catalyzed ring-enlargement reaction. With a silyl group on C3, the rearrangement may be initiated by solvolytic cleavage of the Si —C bond with methanol.90... 

Available methods for the synthesis of macrolides involve the cyclization of long-chain bifunctional precursors,3 depolymerization processes,4 ring-enlargement reactions,5 and special methods... 

Based on the data in the Table and on our published results,7 it is clear that five-, six-, and seven-membered cycloalkenones undergo this 4-atom ring enlargement reaction to produce medium ring, unsaturated lactones in overall yields... 

A versatile palladium-catalysed ring enlargement reaction where arylidenecyclo-propanes are converted into the corresponding cyclobutene compounds has been reported (Scheme 31).90 A plausible mechanism for the unusual ring enlargement... 

Reactions of cyclopropanones with nucleophiles frequently lead to ring enlargement reactions since the formation of four-membered rings from the reactive intermediates is accompanied by a considerable reduction in strain energy. Thus, 2 reacts with diazomethane to form cyclobutanone96>, with hydrazoic acid to form (3-lactam 76,89) and, under special conditions, with amines and hydroxyl amine derivatives to form N-sub-stituted (3-lactams 87> (Scheme 24). 

Alkynyl( aryl) iodine (I II) compounds 89 can also be employed in hetero-Claisen rearrangements [151] after reaction with appropriate thioamides yielding thiazoles of type 90 in reasonable yields as shown in Scheme 38 [152]. Ring enlargement reactions of furan derivatives into pyranones by hypervalent iodine compounds were reported as well [153]. 

The ring enlargement reaction of 3-phospholenes is separately reported by using phase transfer catalyst, then convenient preparation of 1-alkyl-1,2-di-... 

Ring-enlargement reactions of azetoisoquinolines and azeto[l, 2 l,2]pyrido[3,4-4]indole give eight-membered ring compounds which are useful intermediates <1999CORl>. 

Mislin, G.L. and Miesch, M. (2003) Photochemical, thermal and base-induced access to hydroazulene derivatives via two-carbon ring-enlargement reactions of condensed electrophilic cyclobutenes. Journal of Organic Chemistry, 68, 433 141. 

Although there are many different ways to classify ring enlargement reactions, we have chosen a non-uniform approach as shown in the Table of Contents, because this system allows a better incorporation of the references. 

Ring enlargement reactions mediated by metals, silicon, or phosphorous are not treated in this survey because of the tremendous amount of material. Rearrangements of bicyclic compounds with a simultaneous contraction and enlargement of the two rings are also excluded. 

Some miscellaneous ring enlargement reactions are presented in Scheme 11/13 two of them are syntheses of cyclobutanones, 11/90 and 11/92 [71] [72]. 

A number of other ring enlargement reactions proceed via 1,1-dihalocyclopropane intermediates [65] [66]. 

III. The Three-membered Ring - a Building Element for Ring Enlargement Reactions... 

---