Cycloaddition of oxidopyrylium species in organic synthesis Vishwakarma Singh*, Urlam Murali Krishna, Vikrant, Girish K. Trivedi* Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400076, India Contents 1. Introduction ............................................... .........................................3405 2. Generation and cycloaddition of oxidopyrylium species from epoxyindanones .................. .................3406 2.1. Intermolecular cycloadditions .....................................................................3406 2.2. Intramolecular cycloadditions .....................................................................3407 3. Generation and cycloaddition of oxidopyrylium species from acetoxypyranones (pyranulose acetates) .................3407 3.1. Intermolecular cycloadditions .....................................................................3407 3.2. Intramolecular cycloadditions .....................................................................3410 3.3. Asymmetric cycloadditions .......................................................................3411 4. Oxidopyrylium species from b-hydroxy-g-pyrones and their cycloadditions .................... ..................3412 4.1. Intramolecular cycloadditions .....................................................................3412 4.2. Intermolecular cycloadditions .....................................................................3413 4.3. Asymmetric cycloadditions .......................................................................3414 5. Cycloaddition of oxidopyrylium species (from acetoxypyranones and/or b-hydroxy-g-pyranones) in the synthesis of natural products ............................................ ......................................3415 6. Tandem [5þ2] cycloaddition of oxidopyrylium species and [4þ2] cycloadditions .................. ................3420 7. Transformation of dimer of 3-oxidopyrylium ................................. .............................3420 8. Rhodium-catalyzed generation of carbonyl ylides and their cycloadditions .................... ..................3421 9. Concluding remarks ..................................................................................3425 Acknowledgements ............................................ ......................................3425 References and notes .................................................................................3425 Biographical sketch ............................................ ......................................3428 1. Introduction complexity in a stereocontrolled manner are some of the im- portant aspects of organic synthesis.1 Cycloaddition reactions, The development of efficient methodology and the discov- which allow two new bonds to be formed in one operation in ery of new processes for the rapid creation of molecular a regio- and stereocontrolled fashion, occupy a central position Abbreviations: Ac, Acetyl; acac, acetylacetonyl; Bn, benzyl; Bz, benzoyl; COD, cycloocta-1,5-diene; Cp, cyclopentadienyl; m-CPBA, m-chloroperbenzoic acid; Cy, cyclohexyl; DBN, 1,5-diazabicyclo[4.3.0]non-5-ene; DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene; DMAD, dimethyl acetylenedicarboxylate; DMAP, 4-di- methylaminopyridine; DME, ethylene glycol dimethyl ether; DMF, N,N-dimethylformamide; ee, enantiomeric excess; HMPA, hexamethylphoramide; LDA, lith- ium diisopropylamide; Ms, methanesulfonyl; MOM, methoxymethyl; NBS, N-bromosuccinimide; NMO, 4-methyl morpholine N-oxide; PCC, pyridinium chlorochromate; PMB, p-methoxybenzyl; PMP, p-methoxyphenyl; Py, pyridine; PTC, phase transfer catalyst; TBAF, tetra-n-butylammonium fluoride; TBS, tert-butyl dimethylsilyl; TBDPS, tert-butyldiphenylsilyl; TBSOTf, tert-butyldimethylsilyltriflate; Tf, trifluoromethanesulfonate; TIPS, triisopropylsilyl; TMP, tetramethylpiperidide; TMS, trimethylsilyl; TMSOTf, trimethylsilyltriflate; p-Tol, p-tolyl; Ts, p-toluenesulfonyl. 3406 among the available tools of synthetic organic chemistry. the endo isomer and furnished the adducts 14, 15 and 17, 18, Among the various types of cycloadditions, dipolar cycloaddi- respectively (Scheme 3). tions of oxidopyrylium species of the types IeIII (Fig. 1) and the related carbonyl ylides have proved to be a powerful meth- - odology for the synthesis of diverse molecular architectures, O O Ph Ph O which are not readily available otherwise. The intention of Ph Δ or hν O this review is to discuss the developments in the chemistry O O+ 1 Ph 2 of oxidopyrylium species reported in the literature to date. Ph Ph Previously, there have been a few reviews that have appeared 3 MeO C CO Me in the literature, but in most cases, these have dealt with differ- 2 2 2e5 Ph ent perspectives. We wish, in the current survey, to high- COOMe light the cycloaddition of oxidopyrylium ylides and their O O application in organic synthesis. Cycloaddition of the related COOMe carbonyl ylides, generated by rhodium-catalyzed addition of Ph diazo ketones, has recently been reviewed6,7 and hence, only 4 the recent developments are covered here. It is believed that Scheme 1. such an overview will give a better understanding of the sub- ject to the reader and will, hopefully, stimulate further investigation. O Ph X O O O H O O Ph 5a O , X = O X - 5b H O - - 1 , X = NPh Ph O O Ph O Ar OR Δ or 8, X = O (89%) hν 9, X = NPh (93%) O+ O+ O+ O O- RCN Ph I Ar II III Ph O CN 6a, R = H Figure 1. Oxidopyrylium species. O+ 6b, R = Me R 2 Ph Ph 10, R = H (98%) 11, R = Me (80%) 2. Generation and cycloaddition of oxidopyrylium species O Ph Cl Cl from epoxyindanones O Cl 7 2.1. Intermolecular cycloadditions Cl Ph 12 (72%) Ullman and co-workers have reported that 2,3-diphenylin- Scheme 2. denone oxide 1, upon thermolysis or photolysis, produces the red-coloured benzopyrylium oxide 2.8 The benzopyrylium oxide 2 behaves as a carbonyl ylide and undergoes cycload- O O O Ph Ph ditions with alkenes such as norbornadiene and dimethyl ace- H H Δ or hν Ph O O tylenedicarboxylate to produce the cycloadducts 3 and 4, O O + O O O respectively (Scheme 1). Irradiation of 2,3-epoxy-2-methyl- O O 1 Ph O H O O Ph H Ph 3-phenylindanone in the presence of dimethyl acetylenedi- 13 3:1 carboxylate was also found to give an adduct.9 Later, Lown 87% 14 15 and Matsumoto extensively studied the cycloaddition of Ph O H benzopyrylium oxide 2 with various dipolarophiles.10 They O Δ ν O 18 observed that the cis 2p-addends underwent cycloadditions Ph or h + (exo) + O 93% with the benzopyrylium ion 2 to give a mixture of endo PhH 1 Ph 3:2 and exo adducts in which the formation of the endo adducts 16 17 predominated. Scheme 3. The benzopyrylium ylide 2 reacted with alkenes such as maleic anhydride 5a and N-phenyl maleimide 5b and provided exclusively the endo adducts 8 and 9, respectively. Similarly, cycloaddition with cis-alkenes 6a,b gave the adducts 10 and In contrast to the above observations, the cycloaddition of 11, respectively, and 7 also resulted in the formation of the benzopyrylium oxide 2 with cis-stilbene 19 and dimethyl endo adduct 12 (Scheme 2). The alkenes 13 and 16, however, maleate 20 gave a mixture of adducts 21e24 having the exo gave a mixture of endo and exo isomers with a preference for adducts as the major products (Scheme 4). 3407 O O Ph O Ph O Ph Ph Ph Ph Δ ν Ph 19 O Ph O Ph or h O + Ph + O O O 72% Ph O Ph Ph Ph 1 Ph Ph 40 Ph Ph 21 Δ ν 22 41 Ph or h 1:3.6 R1 O Δ O Ph O Ph -CO 1 Ph 94% O Ph O O Ph O CO2Me + CO2Me R2 O R1 CO2Me CO2Me O Ph MeO2C CO2Me Ph Ph 20 23 1:3.6 24 R Ph 2 Ph Ph 43a 42 Scheme 4. , R1 = R2 = COPh (92%) 43b , R1 = Ph, R2 = COOMe (86%) 43c , R1= Ph, R2 = H (82%) e The trans 2p-addends 25 29 also gave a mixture of ad- Scheme 5. ducts 30e39 in which a tendency to form the 2-exo-3-endo isomer is observed (Table 1). With very bulky substituents O such as benzoyl on the addend (entry 5), the exclusive forma- O Ph Δ ν tion of the 2-exo-3-endo isomer was observed. Diphenyl- Ph or h O Bu-t + P Bu-t cyclopropenone 40 underwent cycloaddition with 2 to give O P 44 the cycloadduct 41, which subsequently loses carbon mon- 1 Ph 45 Ph oxide to produce the adduct 42. The adduct 42 was also Scheme 6. obtained directly by the addition of diphenylacetylene to the 10 11a ylide 2 (Scheme 5). George and his associates also 2.2. Intramolecular cycloadditions prepared adducts 43aec by thermal activation of the epoxide 1 in the presence of various acetylenic dienophiles Feldman has reported the intramolecular counterparts of the (Scheme 5). aforementioned cycloaddition.12 Irradiation of the mono- Very recently, Regitz and co-workers reported that unusual substituted epoxyindenones 46 and 48 having olefinic tethers dipolarophiles such as the phospha alkyne 44 also underwent gave the cycloadducts 47 and 49 (Scheme 7). Irradiation of regiospecific cycloaddition to provide an oxa-bridged phospha 11b 48 also produced the product 50, in an equal amount, which alkene derivative 45 (Scheme 6). is apparently formed via the intermediates 51 and 52.12 O O Me hν, 300 nm Table 1 O PhH Cycloaddition of benzopyrylium species derived from 1 with various O Pyrex Me dienophiles 4665% 47 O O O O Ph O Ph O R Δ ν hν, 300 nm Ph or h O O + R O R O PhH Me + O R + O R Pyrex 1 Ph R 48 Me 49 50 Ph Ph 34% Me 25, R = Cl 30, R = Cl 35, R = Cl 36 26, R = CN 31, R = CN , R = CN [2+2] 27, R = Ph 32, R = Ph 37, R = Ph 28 33 38, R = CO Me O , R = CO2Me , R = CO2Me 2 O 29, R = PhCO 34, R = PhCO 39, R = PhCO . O O Entry Dipolarophile Products Ratio Yield (%) 51 Me 52 Me Cl 1 30/35 4:1 69 Cl Scheme 7. CN 2 31/36 7:1 85 NC Ph 3 32/37 5:2