The synthesis of new tricyclic vinylcyclobutanes is reported. These compounds were prepared in good yields from the ketones via a two step Peterson olefination. Under conditions of photoinduced electron transfer some tricyclic vinylcyclobutanes show interesting skeleton rearrangements.
Three tricyclic vinylcyclobutanes (3‐methylenetricyclo[5.3.0.02,6]decanes 1–3) have been subjected to ionization by photoinduced electron transfer in solution and by X‐irradiation in Ar matrices. All three compounds undergo oxidative cycloreversion; the cleavage of the four‐membered ring, however, occurs in a different direction depending on the presence of a methyl group in position 6 of the tricyclic framework. In those derivatives, cycloreversion is found to lead to 1‐methyl‐8‐methylene‐1,6‐cyclodecadiene radical cations (5.+ from 1, 8.+ from 2) which upon back electron transfer yield two different hydrocarbons (6 from 5.+, 9 from 8.+), depending on the configuration around the endocyclic double bonds of the respective cyclodecadiene derivative. In the absence of a methyl group on C6, the cycloreversion leads to a radical cation complex between 1‐methylenecyclopent‐2‐ene and cyclopentene (12.+) which appears to revert to 3 on back electron transfer. The intermediate radical cations 5.+, 8.+, and 12.+ have been identified and characterized by UV/Vis and IR spectra in Ar matrices. The mechanism of their formation is elucidated by quantum chemical calculations.
New tricyclic annelated cyclobutylcarbinols are synthesized diastereoselectively. Acid-induced fragmentation and Wagner-Meerwein rearrangement of these alcohols to give bicyclic dienes and annelated norbornanes are reported.Rearrangement processes and fragmentations of polycyclic hydrocarbon skeletons are frequently used as tools in terpene chemistry. 1 In particular, precursors containing a cyclobutyl moiety are used because they are easily available by photochemical [2+2] cycloadditions 2 and are suitably reactive due to their ring tension. 3 Furthermore, cationic processes, such as the Wagner-Meerwein rearrangement, have been applied as key steps in total syntheses of several terpenes. 4As part of our studies on tricyclic cyclobutane derivatives, we now describe the diastereoselective synthesis of the novel tricyclo[5.n+2.0.0 2,6 ]alkan-3-ols 3a-e, and typical fragmentation and rearrangement reactions of this class of compounds under various acidic conditions. Suitable precursors for the formation of alcohols 3a-e are the tricyclic ketones 2a-d. While tricyclodecanone 2a was obtained by a known procedure, 5 the three diastereomeric 6-methyltricyclo[5.4.0.0 2,6 ]undecan-3-ones (2b-d) were synthesized analogously in moderate yield by photochemical [2+2] cycloaddition of 3-methylcyclopent-2-enone (1) and cyclohexene (Scheme 1). The relative configuration of the cyclobutyl moiety in isomers 2b-d was determined by X-ray crystallography (e.g., ketone 2c, Figure 1) and NMR spectroscopy (e.g., 2d). 6 The nucleophilic addition of Grignard reagents to tricyclic ketones 2a-d to give the corresponding alcohols 3a-e occurs with complete diastereoselectivity in all cases due to the sterical hindrance of the carbonyl group (Schemes 2 and 3). Only one of the two possible diastereomers is formed from each racemic mixture of the ketones 2a-d. The configuration of the new stereocenter was determined by X-ray crystallography for 3c ( Figure 2) and by NOESY experiments for 3a. While the methyl-substituted alcohols 3a and 3c-e are formed in high yields (99 and 90%, respectively), reduction of ketone 2a took place as Figure 1 X-ray structure of 2c Scheme 1 [2+2] Photocycloaddition of 3-methylcyclopent-2-enone to cyclopentene and cyclohexene Downloaded by: University of Queensland. Copyrighted material.
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