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Tetrahedron 63 (2007) 8377–8412 New reaction and new catalyst—a personal perspective Hisashi Yamamoto* Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, IL 60637, United States Received 11 May 2007; revised 22 May 2007; accepted 22 May 2007 Available online 6 June 2007 Abstract—A number of new synthetic methods are reviewed. Most of the methods are based on aluminum, boron, tin, silver Lewis acids and/ or Brønsted acid catalysts. Concepts of combined acid catalysis and super Brønsted acid catalysis are also summarized. These methods are useful for selective organic transformations including simple natural product synthesis. Ó 2007 Elsevier Ltd. All rights reserved. 1. Introduction this lovely science is immortalized. Described herein is a small contribution from our laboratories on organic synthe- Although I am not a good Japanese chess (Shogi) player, sis, particularly regarding the molecular design and engi- I admire one famous professional player, the late Kozo neering of reagents and catalysts. Masuda (1918–1991), the most gifted Shogi player of his era. He became a professional Shogi player while in his youth. After years of practice and dedication, he quickly 2. Aluminum amide: Lewis acid–Lewis base climbed the ranks of the best and became the champion in cooperative system 1957. His popularity did not derive from being an undefeat- able champion; rather what was so special about his game M was that he invented completely novel strategies and tactics O L in his matches. I was amazed by this and indeed I am sure it was not a simple task. Famous professional Shogi players to- day make use of the same conservative strategies in their In my early days, I was interested in synthesizing several games. When people asked Mr. Masuda why he insisted sesquiterpenes from simple and readily available farnesol. on inventing new strategies, his answer was simple: ‘‘I would Although this is not actually a biomimetic route, the inven- like to devote my life solely to creating unbeaten path’’ tion of possible synthetic routes is still quite challenging (shinn te isshou). (Scheme 1). For example, how can we prepare juvenile hor- mone and humulene from farnesol? OH COOMe O Humulene Farnesol Cecropia juvenile hormone Scheme 1. Scheme 2 is one of our answers to this question. A key step of synthesis is a vanadium-catalyzed epoxidation, which re- sulted from fruitful collaboration with Professor Barry Sharpless.1 The reaction proceeds with exceedingly high erythro selectivities. The following steps proceed stereose- lectively: copper alkylation and dehydration to generate I respect his philosophy greatly and feel that his attitude to the key framework. Although the reaction of epoxides Shogi exemplifies the ideal of a synthetic chemist. This with a strong base constitutes a well-known synthetic way of thinking is essential to preserving and making certain method for preparation of the starting allylic alcohols, the in- efficiency of this process led us to develop a new method: the * E-mail: [email protected] aluminum amides for this rearrangement.2 0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2007.05.128 8378 H. Yamamoto / Tetrahedron 63 (2007) 8377–8412 OR OR OR ? O HO O OH OR OR COOMe O OH HO HO O OH HO OH O Cecropia juvenile hormone Scheme 2. The rearrangement proceeded smoothly using bulky alumi- organoaluminum reagents under aprotic conditions led to num amides. Thus, diethylaluminum 2,2,6,6-tetramethylpi- the abstraction of the sulfonyl group, followed by capture peride reacted with the epoxide smoothly and gave the of the intermediary iminocarbocation or alkylidyneammo- allylic alcohols highly efficiently. The observed strict regio- nium ion with the nucleophilic group (X; R2AlX (X¼H, selectivity originated from the stereoselective coordination R, SR0, SeR0)) on the aluminum. Thus, aluminum reagents of a sterically less hindered epoxide lone pair with the nitro- act not only as a Lewis acid but also as a base.4 This method gen of aluminum amide. Thus, the Lewis basicity of nitrogen opens a new synthetic entry to a variety of alkaloids such as was increased significantly by coordination of epoxide to pumiliotoxin C (Scheme 5).5 aluminum. This was a new Lewis acid–Lewis base coopera- tive reaction system (Scheme 3). 1 2 1 2 R N C R R R 1 2 The stereoselective synthesis of humulene is shown in R2AlX X R R N N Scheme 4. The key step of the synthesis is the palladium- OSO R' R1 N C R2 X catalyzed medium ring cyclization, the first transition metal 2 catalyzed cyclization of a medium and a large ring. The base-catalyzed elimination of oxetane to generate homo- allylic alcohol proceeds smoothly using aluminum amide, 3 H H a transformation similar to that described above. 1) n-Pr3Al 2) DIBAL These Lewis acid–Lewis base cooperating systems are N O H N H H H not only effective as an intramolecular system. The inter- OTs 60% molecular version of the process was developed as Pumiliotoxin C follows. Reexamination of Beckmann rearrangement using Scheme 5. N Al H - + Al O O 90% H OH N O 78% OH Scheme 3. 1 LAH O Pd(PPh3)4 O 2 TsCl, base O OAc COOMe COOMe 1 Oxid . Al Et2AlNMePh 2 WK O H N OH Humulene Scheme 4. H. Yamamoto / Tetrahedron 63 (2007) 8377–8412 8379 O R Me S R O O N 1) Et AlCl 2 N O Muscone 2) DIBAL H 88% N Muscopyridine Scheme 6. The intermediary iminocarbocation or alkylidyneammo- These bulky aluminum reagents can be prepared from steri- nium ion generated by an organoaluminum can also be trap- cally hindered phenols. Thus, MAD and ATPH are readily 6 ped intramolecularly with olefinic groups. This interesting prepared by treatment of Me3Al with a corresponding rearrangement–cyclization sequence can be extended to an amount of the phenol in toluene (or in CH2Cl2) at room tem- efficient synthesis of muscopyridine (Scheme 6).7 perature for 0.5–1 h with exclusion of air and moisture. The reactivity of a phenol toward Me3Al largely depends on the stereochemistry of the phenol (Scheme 7). 3. Bulky aluminum reagents The X-ray crystal structure of the N,N-dimethylformamide– ATPH complex11 disclosed that three arene rings of ATPH form a propeller-like arrangement around the aluminum cen- Ph Ph ter, and hence ATPH has a cavity with C3 symmetry. In con- trast, the X-ray crystal structure of the benzaldehyde–ATPH O O Al complex shows that the cavity surrounds the carbonyl sub- O O Ph strate upon complexation with slight distortion from C sym- O 3 Al Ph Ph Ph metry. A particularly notable structural feature of these Me aluminum–carbonyl complexes is the Al–O–C angles and Al–O distances, which clarify that the size and shape of the cavity are flexible and change depending on the sub- strates. According to these models, the cavity should be Most aluminum compounds in solution exist as dimeric, able to differentiate carbonyl substrates, which when ac- trimeric, or higher oligomeric structures. In contrast, cepted into the cavity should exhibit unprecedented reactiv- methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) ity under the steric and electronic environments of the arene (MAD) and aluminum tris(2,6-diphenylphenoxide) (ATPH) rings (Fig. 1). are monomeric in organic solvent. Lewis acidity of these reagents decreases with the coordination of more electron- Selective 1,6-addition of alkyllithiums to aromatic carbonyl donating aryloxides, but this can be compensated for substrates such as benzaldehyde or acetophenone was by loosening of the aggregation. Compared with classical achieved with ATPH to give functionalized cyclohexadienyl Lewis acids, the significant steric effect of our aluminum compounds (Scheme 8).12 According to the molecular struc- reagents plays an important role in selective organic ture of the benzaldehyde–ATPH complex, it is obvious that synthesis.8–10 the para-position of benzaldehyde is deshielded by the three Me3Al (1/2 equiv) 1 R1 OH R O O R1 toluene Al or MAD : R1 = Me CH2Cl2 Me MABR : R1 = Br Ph Ph R2 Ph O Me3Al R2 O Al (1/3 equiv) Ph R2 OH toluene Ph O or Ph Ph ATPH 2 = H Ph CH Cl : R 2 2 ATPH-Br :R2 = Br R2 Scheme 7. 8380 H. Yamamoto / Tetrahedron 63 (2007) 8377–8412 Figure 1. arene rings, which effectively block the ortho-position as ATPH–PhCOCl complex was more reactive than ATPH– well as the carbonyl carbon from nucleophilic attack. Al- PhCHO (Scheme 9).13 though conjugate addition to the ATPH–PhCHO complex did not proceed effectively with smaller nucleophiles, we A similar concept was used in a number of different organic were able to illustrate that ATPH–ArCOCl is superior to transformations, all of which used the selective coordination ATPH–PhCHO for the nucleophilic dearomatic functionali- of Lewis base including carbonyl compounds to ATPH or zation. Several analytical and spectral data showed that the MAD. Several examples are shown in Schemes 10–19. CHO OH + CHO CHO OLi MePh2Si t CO2 Bu OtBu tBuLi Ph2MeSiLi ATPH OLi CHO CHO O 1) Me3Si Li OtBu 2) TBAF/MeOH t CO2 Bu OLi Li Li OMe CHO CHO CHO CO2Me Scheme 8. H. Yamamoto / Tetrahedron 63 (2007) 8377–8412 8381 CO2H The values in parentheses is the ratio of 1,6- and 1,4-adducts. 90%(15:1) CO2H CO2H t-Bu Ph (>99:1) (13:1) i-Pr MgCl PhLi 96% CO2Me 71% MgBr Li (8.5:1) 53% CO2Me OLi Cl Li CO2Me Ot-Bu ATPH O t-BuO 78% 68% (3.4:1) (7.9:1) OLi O CO2H LiO OMe OLi 72% 75% OLi O (>99:1) CO2H 41% 46% CO2H MeO O CO2H (>99:1) O O (>99:1) (>99:1) Scheme 9.
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