Anhydro dimers

Hantzsch190 obtained an anhydro dimer (139) by the action of acetic anhydride or acetyl chloride on j3-oximidobutyric acid anhydride. A spectroscopic study191 showed that the two tautomeric forms (139 and 140a) were predominant the formula 140a is classified as a type D oligomer (see below). 

Isoxazolin-5-ones and pyrazolin-5 ones have been shown to give the anhydro dimers (140a and b, respectively), when treated with a base such as piperidine.191,259... 

The isolation of avenacins A-1, A-2, B-1 and B-2, chemical defences against cereal take-all disease. Structure of the aglycones, the avenestergenins and their anhydro dimers, M.J. Begley, L. Crombie, W.M.L. Crombie and D.A. Whiting, J. Chem. Soc., Perkin Trans 1, 1986, 1905. 

Albert has shown that, under appropriate conditions, a near quantitative yield of a 1,2,3-triazole anhydro dimer (10.1-8) may be obtained and then converted to the important derivative 10.1-9 (Eq. 12)." This compound (10.1-9) has played a role in the synthesis of various azapurines (e.g., 10.1-10), which in turn are promising intermediates in u-triazole synthesis (Eqs. 13,14). Albert has also made important contributions to our understanding of the optimal methods for preparing and isolating various... 

Reaction of compound 56 with Li20 yields the anhydro dimer 57, which is the N,N-dimethyl derivative of anhydro dimer 53, and this provides another route to the anhydro dimers. In analogy with the reaction to form the 1-azonia-2-boratanaphthalene 56, l-oxonia-2-boratanaphthalenes 58a—fare formed by the reaction of phenylethyne with arylchloro(phenyloxy)boranes (Scheme 23). 

With l,3-dimethyl-2,l-benzisoxazolium salts, however, considerable reactivity has been reported. Condensation occurs readily with aldehydes, ketones, orthoesters and diazonium salts to yield styryl, cyanine and azo compounds, respectively (78JOC1233). In the presence of triethylamine, dimerization was observed, and the reactions of the cation were considered to involve the intermediacy of the anhydro base (77JOC3929). 

Oxazolium hydroxide, anhydro-5-hydroxy-aromaticity, 6, 184 cycloaddition reactions, 6, 209 dimerization, 6, 207 1,3-dipolar cycloaddition reactions with alkynes, 6, 210 electrophilic reactions, 6, 207 mesoionic reactions, 6, 188 reactions, 6, 206-211 synthesis, 6, 225-227... 

Note 2. Under acidic conditions and in the presence of water, the 18,20-hemiacetal formed from the 18-iodo-18,20-ether is transformed into a dimeric anhydro product, resistant to further oxidation. Pyridine is added to prevent this transformation. 

The third structural possibility, the formulation of the compounds as pseudo-bases (445) was eliminated in the case of the anhydro-bases derived from p /r-iV -alkyl-l-methyl-3,4-dihydro-j8-carbolinium salts on the basis of their ultraviolet absorption spectra. A structure such as 445 demands indole-type absorption (A jax 280 mp) which was not encountered in the spectra of the anhydro-bases under discussion. This is in accord with general experience. Pseudo-bases are generally found only when dehydration to anhydro-bases is structurally impossible Indole-type absorption was indeed found in the case of the product obtained by treatment of 3,4-dihydro-)3-carboline methiodide (452 R = H) with alkali.In acid solution this compound gave the expected absorption (A jax 355 mp). In alkaline solution, however, an indole-type absorption (A jax 285 mp) was observed. On this basis formulation of the product as a derivative of 2-formylindole (454) ( max 315 mp) was rejected. Although the indole-type absorption is in accord with the pseudo-base structure 453 (R = H), the elemental analysis and molecular weight were not compatible with this formulation and the product was regarded as a dimeric anhydro-base (455). 

This dimeric formulation also accounts for the thermal disproportionation of the compound into a mixture of 2-methyl-l,2,3,4-tetrahydro-jS-carbohne and 2-methyl-jS-carboline anhydro-base. Normal pseudobase formation (453 R = CHs ) takes place in the case of 9-methyl-3,4-dihydro-j3-carbohne methiodide (452 R = CHg). Neither the dimeric anhydro-base nor the -methyl pseudo-base undergo a base-catalyzed disproportionation reaction to give the 1,2,3,4-tetrahydro-and the l-oxo-l,2,3,4-tetrahydro-jS-carboline in a manner analogous... 

Ci.,H2 O10 l,6-Anhydro-2,3-0-isopropylidene-/ -D-arafeino-hexopyranos-3-ulose dimer APARHD 43 238... 

Abe and Prins214 suggested that the first step in the polymerization reaction of levoglucosan consists of a dimerization of all of the levoglucosan units. This involves the opening of all 1,6-anhydro rings, followed by the formation of a reactive, (1— 6)-linked intermediate (52), which polymerizes... 

I. Lundt, J. Marcussen, J. Sotofte, and S. K. Yu, Structure of 1,5-Anhydro-D-fructose X-ray analysis of crystalline acetylated dimeric forms, J. Carbohydr. Chem., 17 (1998) 1027-1035. 

O. R. Martin, and F. Xie, Synthesis and spontaneous dimerization of the tri-O-benzyl derivative of 2-keto-l-C-methylene-D-glucopyranose (2,6-anhydro-4,5,7-tri-0-benzyl-l-deoxy-D-araWno-hept-l-en-3-ulose), Carbohydr. Res., 264 (1994) 141-146. 

The studies of Levene and LaForge60 on the nitration of uridine have since been re-investigated. It had been reported, several decades ago, that treatment of uridine with nitric acid produced a Nitro-Uridin-Carbon-saure which was stated to be a dimer containing an anhydro typeof linkage (positions unspecified) between two nucleoside residues.60 According to an abstract,269 this compound has now been identified as the monomeric 5-ni-tro-l-(/8-D-ribosyluronic acid)uracil (CXXII) this was converted to the isopropyl ester and the 2,3-O-isopropylidene acetal thereof. Nitration of the pyrimidine moiety of uridine without oxidation of the 4-(hydroxy-... 

Anhydro-5-hydroxyoxazolium hydroxides lacking substituents at C(4) dimerize spontaneously by a process in which one molecule acts as an electrophile and the other as a nucleophile (Scheme 21). This accounts for the fact that dimeric products of this type are obtained by the action of dicyclohexylcarbodiimide on acylamino acids of the general formula R1C0NR2CH2C02H. Substituents at position 4 stabilize the mesoionic system the first compounds to be prepared were the acetyl derivatives (220) (B-49MI41800) and (221) (58Cl(L)46l) and much of the more recent work has been carried out with the relatively stable methyldiphenyl compound (222). This miinchnone decomposes above 115 °C to yield the allene (225) with loss of carbon dioxide. The mechanism proposed for this remarkable reaction (Scheme 22) involves valence isomerization to the ketene (223), which undergoes a 1,3-dipolar cycloaddition with the miinchnone. The product loses carbon dioxide to form a new betaine (224), which collapses to the allene as shown. 

---