Ring-opening annulations

Scheme 3.30 Rh(l)-catalyzed ring opening annulation of benzocyclobutenol 53.

Ruthenium-catalyzed ring-opening annulations of cyclobutenediones with norbornene have been reported (Scheme 3.54) [63]. The reaction with 3-alkyl-4-alkoxycyclobutenediones 95a under 3 atm CO gave cyclopentenone 96 via [4-1+2]-type annulation. On the other hand, 3,4-dialkylcyclobutenedione 95b produced a hydroquinone product 97. 

This section contains alkylations of ketones and protected ketones, ketone transpositions and annulations, ring expansions and ring openings and dimerizations. Conjugate reductions and Michael alkylations of enone are listed in Section 74 (Alkyls from Alkenes). 

The 1,3-diene moiety in 227 which included the carbon atoms and CVC was oxidized to the l,4-dihydroxy-2-ene moiety in 238 that was further exploited to functionalise the A-ring as well as for the annulation of the C-ring (Scheme 37). The transformation of 227 into 238 was realized by a diastereoselective epoxidation of 227 to afford a vinyl epoxide (241) that was subjected to the conditions for a Palladium(O)-catalysed allylic substitution with the acetate ion [126]. The mechanism and the stereochemical course of the allylic substitution may be explained as depicted in Scheme 37. Sn2 ring opening of the protonated vinyl epoxide 241 by an anionic Pd complex proceeded with a (3Si) topicity to the r-allyl Pd com-... 

The vinyl silane 247 was unmasked to the ketone 250 by epoxidation, subsequent ring opening of the epoxide with HF in pyridine and concurrent cleavage of the THP and TBS protecting group (Scheme 39). The Stork-Jung annulation was completed through the treatment of the diketone 250 with sodium methoxide to mediate the intramolecular aldol con-... 

Hudlicky and Barbieri (178) developed a simple two-step strategy to generate vinyl oxiranes, and then rearrange the vinyl oxirane to form functionalized dihydrofurans in a formal [2- -3]-annulation process. Both pyrolysis and ring opening with TMSI have been used to initiate the rearrangement process (Scheme 4.90). 

Yokozawa reported that the 2-substituent strongly affects the reactivity of a 2-substituted-oxazoline 360 with dimethyl 2,2-dicyano-3-ethoxy-l,l-dicarboxylate as shown in Scheme 8.115. Thus, oxazoline itself, 360 (R = H), gave the annulated bicyclic product 361 that resulted from collapse of the zwitterionic intermediate 362, whereas simple 2-alkyloxazolines, 360 (R = Me, Et), gave an alternating (1 1) copolymer. 2-Phenyloxazoline, 360 (R = Ph), was unreactive under the reaction conditions. The zwitterionic intermediate 362 (R = Me) was trapped by acetic acid to give the open-chain adduct 363 that resulted from nucleophilic ring opening at the 5-position of the oxazolinium zwitterion. 

The previously described lanthanide triflate promoted ring opening of indoles 115 with 1,1-cyclopropanediesters 116 has been extended to provide an annulation reaction of indoles, wherein nucleophilic ring opening by the indole C-3 followed by attack of the intermediate malonic enolate 117 on the iminium moiety to afford the annulated products 118 <99TL5671>. 

Such disjoint molecules may serve as synthetic intermediates. Thus a [3 + 2] annulation method exploits the facile ring opening of cyclopropanols and the transfer of the donor site to the p carbon of the resulting ketones, the nascent carbanions may... 

Attempted annulation of 3-iodoflavones, for example 428, by internal alkynes catalyzed by Pd(OAc)2 in the presence of NaOAc and LiCl yielded a mixture of two compounds (Scheme 73) <1998JOC2002>. While the expected benzoxanthenes (e.g., 429) were isolated as the minor compound, tetrasubstituted furans (e.g., 430) resulting from the electrophilic attack on the carbonyl oxygen atom by the vinylpalladium intermediate followed by pyrone ring opening were the major product. 

Cha s group [89] has recently developed a convenient method for the ring opening of tertiary aminocyclopropane tethered with an alkenyl moiety 199. The PET reaction of 199 led to formation of the five-membered bicyclic system 200 through a [3 + 2] annulation reaction in a good yield, as shown in Scheme 8.55. 

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