Heteroannulenes

The commonest of these for oxirane opening are amines and azide ion [amide ions promote isomerization to allylic alcohols (Section 5.05.3.2.2)]. Reaction with azide can be used in a sequence for converting oxiranes into aziridines (Scheme 49) and this has been employed in the synthesis of the heteroannulenes (57) and (58) (80CB3127, 79AG(E)962). 

Heteroannulenes electrophilic reactions, 7, 726 tr-excessive synthesis, 7, 727 nucleophilic reactions, 7, 727 Hetero[l IJannulenes structure, 7, 715 Hetero[ 12]annulenes pyridine-like methano-bridged, 7, 715 Hetero[ 13]annulenes unrestricted structure, 7, 716... 

Thiepin, as a seven-membered conjugated system with sulfur as heteroatom, is a member of the 8 7t-electron heteroannulenes which are antiaroinatic according to Hiickel s rule. In contrast to oxepin, thiepin is not stable at room temperature and no valence isomerism with an arene sulfide has been observed. Stable thiepins are obtained only when two bulky substituents, e.g. /ert-butyl, are introduced into positions 2 and 7. In benzothiepins the annellation effect of the aromatic rings contributes decisively to the stability of these compounds stability increases with an increasing number of fused benzene rings. 

Hiberty, P. C. The Distortive Tendencies of Delocalized n Electronic Systems. Benzene, Cyclobutadiene and Related Heteroannulenes. 153, 27-40 (1990). 

Eor reviews of 76 (X = N) and other nine-membered rings containing four double bonds and a hetero atom (heteronins), see Anastassiou, A.G. Acc. Chem. Res., 1972,5, 281, Top. Nonbenzenoid Aromat. Chem., 1973,1,, Pure Appl. Chem., J975, 44, 691. For a review of heteroannulenes in general, Anastassiou, A. G. Kasmai, H.S. Adv. Heterocycl. Chem., 1978, 23, 55. 

Seven-membered conjugated systems having a hetero atom, heteropin (7), consist of 177-azepine (7, X = NH), oxepin (7, X = O), and thiepin (7, X = S). Since these divalent hetero atoms are isoelectronic with the ethylenic linkage, heteropins might be considered to be 8tt electron heteroannulenes which are not aromatic but antiaro-... 

Hiberty PC (1990) The Distortive Tendencies of Delocalized tt Electronic Systems. Benzene, Cyclobutadiene and Related Heteroannulenes. 153 27-40 Hintz S, Heidbreder A, Mattay J (1996) Radical Ion Cyclizations. 177 77-124 Hirao T (1996) Selective Transformations of Small Ring Compounds in Redox Reactions. 178 99-148... 

Because of the enhanced rigidity of the higher-membered heteroannulenes, e.g. (36), imparted chiefly by the presence of properly positioned trans double bonds, these frames are generally less thermally labile than their heteronin counterparts although here too aromatic derivatives are substantially more heat resistant than their polyenic relatives. It was noted, for example (75PAC(44)69l), that while parent aza[13]annulene (37a) is thermally stable at 56 °C, its polyenic acetamide (36a) readily rearranges ( i/2< 1 h) under these conditions to what is believed to be a tricyclic isomer (96). 

The electrophilic reagents which the various heteroannulenes have been deliberately exposed to, thus far, are substances with strong affinity for the heteroatomic lone pair and have been employed uniformly for purposes of heteroatom functionalization. The reactions may be divided into the three related categories listed in Scheme 7. 

Exposure of the heteroannulenes to nucleophiles has been limited largely to the purpose of stripping a suitably N- substituted azaannulene of its substituent (78AHC(23)55). Thus, nine-, 13- and 17-membered urethanes (2a), (5a), (35), (36b) and (59)-(61) were readily converted to the corresponding azaannulenides on exposure to potassium t- butylate at subzero temperatures while 13-membered acetamide (36a) afforded the desired anion (37b) on mild subzero treatment with methyllithium. 

All available heteronins (78AHC(23)55), polyenic as well as aromatic, yield the fully saturated counterparts upon exposure to catalytic (Rh/C) hydrogenation at atmospheric pressure (72ACR281), and there is every reason to believe this to be the typical response of other known heteroannulenes as well. 

Given the availability of a suitable chromophore, the use of light offers the mildest possible thermal environment in which to effect a synthetic transformation insofar as photoinduced processes may be activated at temperatures well below 0 °C. Not unexpectedly then, one finds the photosynthetic procedure to be, thus far, the only one which has been successfully applied to the construction of the relatively sensitive parent (unrestricted) 77- excessive heteroannulenes as shown in Scheme 9a. Similarly, exposure of the tetracyclic azides depicted in Scheme 9b to low-pressure mercury irradiation leads to deazotation and the formation of the pyridine-like 14- and 18-membered parents (49) and (75). 

A notable characteristic of the entire group of unrestricted heteroannulenes displayed in Scheme 9 is that one may trace the synthetic origin of each individual member to the same basic synthon, namely the hydrocarbon cyclooctatetraene. 

Thermal bond transposition has been utilized successfully in the synthesis of a number of relatively heat-insensitive families of heteroannulenes, such as the triheteronin frame (20) constructed thermally from the tetracyclic valence tautomer and the tetrabenzo heteronins (18) obtained by thermal bond transposition of their spirostructured valence tautomers (113). Similarly, thermally induced bridge extrusion (C02) has been utilized as a means of preparing dihydrobenzazonine (112) (the direct photoprogenitor of aza[13]annulene 36a) from cycloadducts (104) (73TL3805). 

Finally it is noted that while the conditions associated with conventional Hoffmann eliminations are obviously much too drastic for preparing heteroannulenes, a rather clever modification of the procedure (one involving the rupture of a cross-link) was successfully applied more than a decade ago to the synthesis of the fully saturated diazonine (120) and its mostly saturated benzo counterpart (121) as well as the partially saturated diazecine (122) and its dibenzo counterpart (123) (69JOC2715, 2720). The fully saturated counterparts of the molecules shown in (121)-(123) were also prepared in that early study. 

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