D. Rajagopal

Diastereofacial selectivity in Diels-Alder cycloadditions of methyl acrylate to facially differentiated unsymmetrical Cyclohexa-1,3-dienes

Abstract The Diels-Alder cycloadditions of methyl acrylate to the unsymmetrical hexacyclo[10.2.1.02,11.04,14.09,13pentadecadienes 1b–f proceed with a high degree of π-facial stereoselectivity but only moderate regioselectivity. The results of fixed model transition state calculations, performed in an effort to establish the factors which control selectivity in these cycloadditions, are reported. Diels-Alder cycloadditions of methyl acrylate to the unsymmetrical hexacyclo[10.2.1.02,11.04,9.04.14.09,13]pentadecadienes 1b–f have been studied, and in each case the resulting [4 + 2] cycloadducts have been fully characterised. The addition reactions proceed with a high degree of π-facial stereoselectivity but with only a moderate level of regioselectivity. The observed π-facial selectivity is correctly predicted from BAE distributions of the ensemble of conformations from a conformational search employing a fixed model transition state. However for 1c, the model predicts that top face addition becomes marginally competitive and this is not reproduced in the experiment. The regioselective trends observed for bottom face addition are predicted similarly from BAE distributions of the ensemble of conformations from a conformational search employing a fixed model transition state. However, the balance of electrostatic vs. steric factors is not well reproduced for the small energy differences involved in the regioselection for the Diels Alder cycloadditions to 1b and 1c. Further insight into the subtle blend of steric and electronic factors which influence the regioselectivities of these Diels-Alder reactions awaits detailed theoretical analysis. Computational approaches designed to clarify these points are under way in our respective laboratories.

Generation and trapping of a caged cyclopentylidenecarbene

The reactive intermediate that is produced both (i) via reaction of 8-(dibromomethylene)-pentacyclo[5.4.0.02,6.03,10.05,9]unndecane (6) with n-BuLi-THF and (ii) via the corresponding reaction of pentacyclo[5.4.0.02,6.03,10.05,9]undecan-8-one (11) with diethyl diazomethylphosphonate (DAMP) has been shown to be vinylidenecarbene 7a rather than the corresponding cycloalkyne, 7b.

Highly regioselective hypervalent iodine mediated ring cleavage and ring expansion reactions of some pentacyclo[5.4.0.02,6.03,10.05,9]undecane derivatives

Abstract Iodosylbenzene-iodine mediated oxidative fragmentation of 3-hydroxy-4-oxa-5-methyl-hexacyclo[5.4.1.02,6-.03,10.0{5,9}.0{8,11}]dodecane (1) proceeds with regioselective C(3)-C(10) σ-bond cleavage to afford exo-10-iodo-4-oxa-5-methylpentacyclo[5.4.1.02,6.05.9.08.11]dodecan-3-one (4) in 94% yield. The corresponding reaction of 1-hydroxy-12-oxapentacyclo[5.4.1.02,6.0{3,10}.05,9]dodecane (6) proceeds with regioselective C(1)-C(11) σ-bond cleavage, thereby affording endo-9-(iodomethyl)-5-oxatetracyclo[6.3.0.02,6.03.10]undecan-4-one (9, 94% yield). Treatment of exo-8-(bromoethynyl)-endo-8-hydroxypentacyclo[5.4.1.02,6.03,10.05,9]undecane (12) with PhI(OH)(OTs)-12 resulted in highly regioselective ring expansion to afford 8-[(Z-)bromoiodomethylene]pentacycl[5.4.0.02,6.03,11.05,10]dodecan-9-one (13) as the exclusive reaction product (67% yield). The structures of 4, 9, exo-8-ethynyl-endo-8-hydroxypentacyclo[5.4.1.02,6.03,10.05,9]-undecane (11), and 13 were established unequivocally via single crystal X-ray structural analysis.

Reactions of some sulfur(II)- and iodine(III)-containing electrophiles with endo-tricyclo[6.2.1.02,7]undeca-4,9-diene-3,6-dione

Abstract Reaction of ArSCl (Ar = phenyl or ortho-nitrophenyl) withendo-tricyclo[6.2.1.02,7]- undeca-4,9-diene-3,6-dione (1) results in anti addition across the norbornene carbon-carbon double bond with concomitant aromatization of the cyclohexenedione ring, thereby affording 4 (87%) and 5 (63%), respectively. The corresponding reaction of PHSCl with 1 in the presence of added Ag(I) proceeds via polar addition across the norbornene carbon-carbon double bond with concomitant intramolecular nucleophilic trapping of an intermediate episulfonium ion by a distant CO group, thereby affording 6 (57% yield). The reaction of PhICl2 with 1, when carried out in the presence of SbCl5 or AgBF4, proceeds with concomitant Wagner-Meerwein rearrangement and aromatization to afford adducts 7a and 8, respectively.

SYNTHESIS OF EXO-5-AZIDONORBORNENE AND EXO-2,EXO-5-DIAZIDONORBORNENE

Abstract Polar addition of HN3 to norbornadiene (1) affords exo-5-azido-norbornene (2, 70%). Subsequent azidomercuration-demercuration of 2, performed by using in situ generated Hg(N3)2 followed by reductive demercuration, proceeds stereospecifically to afford exo-2,exo-5-diazidonorbornane (5, 68% yield).

Structure of a PCU-derived pinacol formed via sodiumpromoted reductive dimerization of pentacyclo[5.4.0.02,6.03,10.05,9]undecane-8,11-dione monoethylene ketal

The pinacol resulting from sodium promoted reductive coupling of pentacyclo[5.4.0.02,6.-03,10.05,9]undecane-8,11-dione monoethylene ketal possesses a conformation, dictated by intramolecular hydrogen bonding, that lies midway between eclipsed and staggered. The C−C and C−O distances in the pinacol unit are, respectively, longer and shorter than usual, and all parameters are as expected.

Tetracyclo[6.3.0.04,11.05,9]undecane-2,7-dione

AbstractThe compound crystallizes in the triclinic space group,

A NOVEL APPROACH TO THE SYNTHESIS OF 1,3,3-TRINITROAZETIDINE

P\bar 1