Benzo-fused bases

Another example comes from the benzo-fused derivatives of nucleic acid bases, which have been conceived of as potential building blocks to develop modified DNA duplexes with nanotechnological applications [74], These benzo-fused bases consist of a benzene unit inserted between the six- and five-membered rings of purines, and of the same... 

This type of ring interconversion is represented by the general expression shown in Scheme 15. Analogous rearrangements occur in benzo-fused systems. The known conversions are limited to D = O in the azole system, i.e. cleavage of the weak N—O bond occurs readily. Under the reaction conditions, Z needs to be a good nucleophile in its own right or by experimental enhancement (base catalysis, solvent, etc.) and Z is usually O, S, N or C. 

The synthesis of this series involved the reaction of disubstituted or benzo fused 6-keto(formyl)-2-cyclohexenones with hydroxylamine (Scheme 176), Base degradation gave a-cyanoketones which can be further degraded to diacids (67AHC(8)277, 80IJC(B)406). 

Substituted benzo[b]furan can be oxidatively converted to the lactone 57 in 81% yield, which is allowed to react with chloroformate containing a chiral trans-( R,2R)-2-phenylcyclohexyl group (R ) to give the benzo[ft]furan-based enol carbonate. When the carbonate is treated with DMAP, nearly complete conversion to the new benzo-fused y-lactone 58 is observed in dichloromethane or in THF with a diastereomeric ratio of 3 1 <00OL1031>. 

By contrast to the bases, the cationic species have long been known. In addition to Fozard and Jones bicyclic salts already described (307, 308), benzo-fused derivatives were synthesized in the context of the aromatic cyclodehydration series by Bradsher s group (Scheme 71). Different from the 2-propenylpyridinium salts 316 just mentioned, the cycHzation of 2-benzylpyridinium 318 salts does not give indolizines unless under basic conditions in the acidic medium morphanthridizinium salts, such as 319a and b, are formed (59JA2547). This cyclization (after iodo-chloro metathesis) takes place in several days but in the case of the... 

Such a reaction may occur on treatment of the benzo-fused bicyclic sulfonium salt 37 with various bases under aerobic conditions. The incipient allylic diradical 39 is trapped as 1,3-diradical by molecular oxygen to give a mixture of up to 35% diastereomeric Spiro-1,2-dioxolanes 40. The spirocyclopentene 41 is obtained in a competing path in which allylic diradical 39 is cyclized as a 1,5-carbon-centered diradicaE . 

The N-2 atom in l,2-dihydro-l-methyl-2,3,l-benzodiazaborine can easily be methylated by DMS in the presence of potassium hydroxide. This base is not sufficiently strong to be used in (V-alkylation of the nitrogen in benzo fused l,2-dihydro-l,2-azaborines in this case organolithium compounds can be employed. By successive treatment of (177 R = H) with methyllithium and an alkyl halide a number of A-alkyl derivatives have been prepared. 

In the benzo-fused series, synthetic efforts have given rise not only to new quinazolines, but also to new methods of preparing them. Quinazoline 34 has been obtained via deprotonation of A-phenyldiethylketenimine with excess strong base (LTMP/KOrBu) in rert-BuOMe followed by addition of another equivalent of the ketenimine and subsequent introduction of rBuCOCl [95T9031]. Semiempirical computational analyses were conducted to provide support for the... 

Reaction of the naphthalene-based a,(3-unsaturated thioketone 478 with various acrylate dienophiles leads to the benzo-fused thiochromans 479 from which elimination of the phenylthio group, which may require the addition of base, affords the benzo-fused thiochromenes (Scheme 168) <1992BCJ2056>. These latter products are themselves suitable substrates for hDA reactions which produce dithiabenzo[c]chrysenes 480 <1994BCJ2876>. 

The pyrilium cation 9.1, 2-pyrone 9.2, 4-pyrone 9.3, and their benzo-fused analogues the benzopyrilium cation 9.4, coumarin 9.5, chromone 9.6, are the parent structures of a series of six-membered ring heterocycles containing one oxygen atom. The impetus for research in this area comes from the enormous number of plant-derived natural products based on the benzopyrilium, coumarin, and chromone structures. 

A short two step approach to the synthesis of l-sulfonyl-l,4-diazepan-5-ones and of their benzo-fused derivatives 129 was developed, based on an Ugi multi-component reaction followed by a subsequent intramolecular Mitsunobu Sw2 reaction <07JOC2151>. 

Aryl-substituted and benzo-fused 1,4-dithiins 624 and 626 underwent a base-induced rearrangement to afford 1,3-dithioles (Equations 84 and 85) <2001TL875>. 

Modeling studies based on a template structure constructed from the superposition of the energy-minimized benzo-fused ACE inhibitors shown in Figure 16.42 (compounds 27 and 29) suggested the synthesis of the 13-membered heterocyclic lactam analog (compound 28). ... 

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