Heterocyclics with 2 different hetero

Selenazole is the selenium-containing compound in the series of heterocyclic 5-membered ring azoles with two different hetero atoms, of which the first two members are oxazole and thiazole. The numbering of the ring system is according to the scheme given (1). 

An efficient, practical solid-phase synthesis of a variety of bis-hetero-cyclic compounds was developed starting from resin-bound orthogonally protected lysine (Fig. 10). Tetraamines 36 were synthesized by exhaustive reduction of resin-bound tetraamides 35. Cyclization with different commercially available bifunctional reagents such as cyanogen bromide, thio-carbonyldiimidazole, carbonyldiimidazole, and oxalyldiimidazole yielded the corresponding bis-heterocyclic compounds bis-cyclic guanidines 37,39 bis-cyclic thioureas 38, bis-cyclic ureas 39, and bis-diketopiperazines 40, respectively.40 Reduction of compounds 40 led to bis-piperazines 41. 

H- 2] Cycloaddition is one of the most convenient methods for the construction of optically active four-membered carbon- or heterocyclic rings that are useful intermediates in organic synthesis. Titanium Lewis acids have been found to promote formal [2 + 2] cycloaddition between two olefins with different electronic features [181]. The analogous hetero [2 + 2] addition of a chiral aldehyde to a silylketene can be catalyzed by TiCU to give the propiolactone with high diastereoselectivity as shown in Scheme 14.80. The silyl group in the product can be easily removed by treatment with KF [182]. 

Interestingly, when the chloro analog was transmetallated and treated with 3-ethoxy cyclohexen-l-one, the expected enone (XI) was not observed, but an enone with a mass of 34 units greater than (XI) was noticed. It also indicated the enone carried the chloro analog. It was presumed that the hetero atoms in the heterocycle present in the starting material (VIII) had performed a directed metallated lithiation providing a different enone bearing the chloro moiety. 

By using a different length of tether between the aldehyde and the dienophile moiety in the aromatic or hetero-aromatic substrates various different highly diversified heterocyclic compounds can be prepared. Thus, reaction of 19 and 15b led to 20 containing a new 5,6-ring system, whereas reaction of 21 and 15b gave 22 with a 7,6-ring system (Scheme 5.5) [11],... 

Abstract This chapter is devoted to recent progress in the chemistry of the 5 5 fused heterocyclic systems. There are four possible modes of 5 5 fusions of the simple five-membered heterocycles leading to four structures containing one heteroatom in each ring. The heteroatoms may be the same or different and may be O, NH, S, Se, Te, P, As, or Sb. The fully conjugated hetero analogs of pentalene dianion have a central C-C bond and are isoelectronic with the 10-7t-electron pentalene dianion. The scope of the chapter is outlined with a survey of various structural types and nomenclature of the parent compounds and their derivatives. New synthetic procedures and synthetic applications of title compounds are presented. This review has concentrated on the new developments achieved from 1997 to September 2007. 

We are familiar with heterocyclic ring systems with various elements, ring sizes, and numbers of hetero atoms. Here only systems with Si-Si bonds are discussed. The number of possible systems is very great in this field with the frequent possibility that different isomers are present. Only a small number of such ring systems have so far been synthesized. 

Most importantly, the scope of the Diels-Alder reaction is very high - not only allowing the synthesis of cyclohexenes and 1,4-cyclohexadienes using 1,3-butadienes and alkenes and alkynes, respectively, but also giving access to a multitude of different heterocycles by exchanging the atoms a-d in the butadiene as well as the atoms e and f in the alkene by hetero atoms such as oxygen, nitrogen and sulfur. However, also dienes and dienophiles with several other atoms as phosphorous, boron, silicone, and selenium have been described. Thus, many different heterodienes and heterodienophiles have been developed over the years (Tables 1-1 and 1-2). 

Under conditions of irreversible deprotonation (BuLi in THF or Et20), the 2-position of the hetero ring is metallated [1, 2, 9]. The pK values of benzofuran and benzothiophene [124] are slightly lower than those of the non-condensed heterocycles, so that complete metallation in the 2-position presumably can be effected with LDA in THF. However, the difference in acidity between the 2- and 3-protons is probably less than that in furan and thiophene. Ring opening therefore seems relatively easy under suitable basic reaction conditions (compare [141]), e.g.) ... 

As with the azoles, oxa- and thiadiazoles are very weak bases due to the inductive effects of the extra hetero atoms, although A-quaternisation reactions can be carried out. For similar reasons, electrophilic substitutions on carbon are practically unknown, apart from a few halogenations and mercurations - it is an intriguing paradox that mercurations, with what is generally thought of as a weak electrophile, are often successful in electron-poor heterocycles. Another important difference from the azoles is of course the absence of A-hydrogen, so that A-anion-mediated reactions are not possible. 

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