Structural Investigations into the retro-Diels-Alder Reaction. Experimental and Theoretical Studies

Abstract

The manifestations of the retro-Diels Alder reaction in the ground-state structures of a range of cyclopentadiene and cyclohexadiene cycloadducts 9-15 have been investigated by a combination of techniques. These include low-temperature X-ray crystallography, density functional calculations (B3LYP/6-31G(d,p)) on both the ground states and transition states, and the measurement of (13)C-(13)C coupling constants. We have found that the carbon-carbon bonds (labeled bonds a and b), which break in the rDA, are longer in the cycloadducts 9-15 than in their corresponding saturated analogues 9s-15s, which cannot undergo the rDA reaction. The degree of carbon-carbon bond lengthening appears to be related to the reactivity of the cycloadduct, thus the more reactive benzoquinone cycloadducts 5b and 13 have longer carbon-carbon bonds. Those cycloadducts 14 and 15 which are predicted to undergo asynchronous reactions show differing degrees of carbon-carbon bond lengthening, reflecting the differing degrees of bond breaking at the calculated transition states for the rDA.

The existence of secondary orbital interactions

“…We obtained 1.3 kcal/mol (in isolation) at B3LYP/6‐311+G**, and our endo ‐(TS(CPD malanhyd) endo ) and the exo ‐TS (TS(CPD malanhyd) exo ) geometries (Fig. 9) are similar to those reported earlier 11, 34. According to Cossio's detailed analysis using second‐order perturbation theory, “SOI do exist and are responsible for at least an important part of the observed stereocontrol.” The computed NICS and Λ (Table 2) support the existence of SOI in TS(CPD malanhyd) endo .…”

Reaction of N-substituted exo-2-hydroxy-5-oxo-4-oxatricyclo[4.2.1.03,7]nonane-endo-9-carboxamides with acetic acid

“…The six-membered C 1 C 2 C 3 C 4 C 5 C 6 ring adopts a boat conformation with the following puckering parameters [11]: S = 1.13, θ = 87.4°, ψ = 1.6°. Such configuration of the cage-like fragment in imide IIIc is typical of analogous compounds [12,13].…”

“…The heterocyclic fragment is oriented endo relative to the bicyclic skeleton [the torsion angle C 6 C 4 C 5 C 9 is -124.1(2)°]. The imide nitrogen atom has a planartrigonal bond configuration [the sum of the bond angles at the nitrogen atom is 360(1)°], as in analogous structures [12,13]. The N 1 -C 14 bond length is 1.429(5) Å, which is typical of unconjugated bonds (average bond length 1.426 Å [15]), and the aryl group is turned through an angle of 81.3(5)° [torsion angle C 8 N 1 C 14 C 15 ] with respect to the heteroring, indicating the absence of conjugation between the imide fragment and the aromatic ring.…”

Synthesis and Biological Evaluation of Novel Tricyclic Pyrrolidinyl (R)‐Alcohols and Amines

“…Figure shows the synthesis of chiral pyrrolidinyl alcohols 1a – c . The key template 3a – c for synthesizing the target molecule la – c was synthesized according to the methods given in the literature . Tricyclic pyrrolidinyl alcohols 1a – c were conveniently synthesized from R ‐(‐)‐2‐ fenil glisinol via two step reaction: imitation and subsequent reduction of carbonyl groups with LiAIH 4 .…”