Functional [6]Pericyclynes: Synthesis through [14+4] and [8+10] Cyclization Strategies

Abstract

Critical analysis of possible strategies for the synthesis of novel carbo-benzene derivatives suggests several [(18-n)+n] routes for the preparation of hexaoxy[6]pericyclyne precursors. Beyond the previously attempted [9+9] symmetrical scheme (n=9), the a priori most selective strategies are those for which n=1, 4, 7, 10, 13, and 16. They involve a cyclizing double-propargylation of a C(18-n) omega-bis-terminal-skipped oligoyne containing (19-n)/3 triple bonds with a C(n) omega-dicarbonyl-skipped oligoyne containing (n-1)/3 triple bonds. To complement the previously used [11+7] strategy, the [14+4] and [8+10] strategies were thus explored. They proved to be efficient, affording seven novel hexaoxy[6]pericyclynes corresponding to six different substitution patterns. These compounds were obtained in 7-15 steps as mixtures of stereoisomers. Thus, by using dibenzoylacetylene as the C(4) electrophile, a [14+4] strategy allowed the synthesis of two hexaphenyl representatives with two or four free carbinol vertices. This approach also afforded tetraphenyl representatives in which the two remaining carbinoxy vertices were substituted with either two alkynyl or one 4-anisyl and one 4-pyridyl groups. By using the hexacarbonyldicobalt complex of butynedial as the C(4) electrophile, a [14+4] strategy also allowed the isolation of a tetraphenylhexaoxy[6]pericyclyne with two adjacent unsubstituted carbinol vertices. A regioisomer with two opposite unsubstituted carbinol vertices was obtained through an [8+10] strategy and its oxidation afforded the corresponding pericyclynedione. Several attempts at synthesizing diphenylhexaoxy[6]pericyclynes with four unsubstituted carbinoxy vertices are described. Only an [8+10] strategy allowed the generation of a fragile diphenylhexaoxy[6]pericyclyne with four adjacent unsubstituted carbinoxy vertices, which could be partly characterized. These results show that the synthesis of the nonsubstituted hexahydroxy[6]pericyclyne, the ring carbo-mer of [6]cyclitol, is a difficult challenge.

Functional [6]Pericyclynes: Aromatization to Substituted carbo‐Benzenes

“…Their intrinsic chemical value is limited by the fact that they were obtained as mixtures of stereoisomers. [10] The stereochemical disorder can however be cancelled by converting stereogenic sp 3 vertices to sp 2 vertices, either partly through oxidation of secondary carbinol vertices (as in the pericyclynedione 3 f from pericyclynediol 3 e), [10] or completely through reductive aromatization. The procedure of Kuwatani, Ueda, and co-workers for the generation of aryl-substituted carbo-benzenes of three-fold "meta" symmetry 1 a-d (Scheme 1) [7] from the [6]pericyclynes 3 a-e,g is envisioned here.…”

“…These authors prepared this compound by reductive aromatization of the [6]pericyclynetriol 5 c possessing alternating C(Ph)OH and C(Ph)OMe vertices. [10] The applicability of the aromatization process has thus been attempted with [6]pericyclynetetrol 3 a and [6]pericyclynediol 3 a', both possessing adjaScheme 1. Top: Delocalized representation of the superimposition of the KekulØ resonance forms of the carbo-benzene rings.…”

“…[10] In order to facilitate reading of the two papers, the same compound numbering has been adopted in both reports. Note also that 4 a and 1 a are the same molecule, namely hexaphenyl-carbo-benzene.…”

Ring carbo-mers: From questionable homoaromaticity to bench aromaticity

“…2) [16,17]. These molecules can be regarded as derivatives of carbo- [N]cyclitols (N = 5,6), for which many substitution patterns have been exemplified: the vertices can be sp 3 -secondary (C(H)OR, R = H, Me), sp 3 -tertiary (C(R')OR, R' = Ph, 4-An, 4-py, C≡C-SiR" 3 ), sp 3 -masked carbonyls (ketal), sp 2 -carbonyls, or sp 3 -heteroatomic (P(O)Ph, SiPh 2 ).…”

Hexasilylated Total Carbomer of Benzene

“…[2a, 4] Since then, all reported carbobenzene derivatives bore aromatic substituents to enhance their stability by radial electron delocalization; however, the possibility of combining aromatic and non-aromatic substituents was also demonstrated, as well as the possibility of the presence of adjacent nonsubstituted vertices. [5] Encouraged by the preparation of a dialkynyl derivative, [5b] we envisioned the preparation of the hexaalkynyl carbobenzene 1 a, which is a protected version of the total carbomer 1 b and was studied recently at the level of density functional theory (DFT; Scheme 1). [6] The target 1 a is also a skeletal carbomer of hexa(trialkylsilyl)benzenes, such as 1 a' [7] and 1 a''.…”