Dirhodium caprolactamate [Rh2(cap)4] is a highly effective catalyst for the oxidative Mannich reaction. The reaction proceeds via C-H oxidation of a tertiary amine followed by nucleophilic capture. This green transformation is conducted in protic solvent using inexpensive T-HYDRO (70% t-BuOOH in water). Synthetically valuable gamma-aminoalkyl butenolides are obtained.
Two new diterpene glycosides in addition to five known glycosides have been isolated from a commercial extract of the leaves of Stevia rebaudiana. Compound 1 (rebaudioside KA) was shown to be 13-[(O-β-d-glucopyranosyl)oxy]ent-kaur-16-en-19-oic acid 2-O-β-d-glucopyranosyl-β-d-glucopyranosyl ester and compound 2, 12-α-[(2-O-β-d-glucopyranosyl-β-d-glucopyranosyl)oxy]ent-kaur-16-en-19-oic acid β-d-glucopyranosyl ester. Five additional known compounds were identified, rebaudioside E, rebaudioside M, rebaudioside N, rebaudioside O, and stevioside, respectively. Enzymatic hydrolysis of stevioside afforded the known ent-kaurane aglycone 13-hydroxy-ent-kaur-16-en-19-oic acid (steviol) (3). The isolated metabolite 1 possesses the ent-kaurane aglycone steviol (3), while compound 2 represents the first example of the isomeric diterpene 12-α-hydroxy-ent-kaur-16-en-19-oic acid existing as a glycoside in S. rebaudiana. The structures of the isolated metabolites 1 and 2 were determined based on comprehensive 1D- and 2D-NMR (COSY, HSQC, and HMBC) studies. A high-quality crystal of compound 3 has formed, which allowed the acquisition of X-ray diffraction data that confirmed its structure. The structural similarities between the new metabolites and the commercially available stevioside sweeteners suggest the newly isolated metabolites should be examined for their organoleptic properties. Accordingly rebaudiosides E, M, N, O, and KA have been isolated in greater than gram quantities.
A fructose-derived diacetate ketone has been shown to be an effective catalyst for asymmetric epoxidation. High ee's have been obtained for a variety of trans-and trisubstituted olefins including electron-deficient α,β-unsaturated esters as well as certain cis-olefins.Chiral ketones of various structures have been extensively investigated for asymmetric epoxidation of olefins in a number of laboratories. 1 In our studies, we have found that fructosederived ketone 1 provides high ee's for a wide variety of trans-and trisubstituted olefins, 2 and oxazolidinone ketone 2 can give high ee's for olefins which had not been effective with ketone 1, including various cis-olefins, 3a,c,d,e,g,k,l styrenes, 3b,c,d,f and certain trisubstituted 3h,j and tetrasubstituted olefins. 3i,j In our efforts to expand the substrate scope, we have reported that ketone 3a is an effective epoxidation catalyst for a variety of electron-deficient α,β-unsaturated esters. 4 Replacing the fused ketal of 1 with more electron-withdrawing diacetates significantly enhances the ketone's reactivity and possibly reduces the Baeyer-Villiger decomposition. Herein we wish to report our detailed studies on epoxidations with ketone 3a and its related analogues.Email: yian@lamar.colostate.edu. Ketone 3a can be synthesized from ketone 1 in two steps by selective deketalization with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) 5 and subsequent acetylation with Ac 2 O and catalytic amount (2 mol%) of DMAP (Scheme 1). 4 Ketone 3a is highly electrophilic and can easily form a hydrate with water. 6 With this method, ketone 3a can be obtained largely in ketone form by avoiding moisture. The β-acetate of ketone 3a is prone to elimination to form an enone. To minimize this elimination, it is crucial to use only a small amount of DMAP (ca. 2 mol%) and to carefully control the reaction time in the acetylation step. Nevertheless, it was found that the resulting enone was barely active and had little impact on the asymmetric epoxidation. Alternatively, ketone 3a can be readily prepared in hydrate form from ketone 1 by one pot deketalization and acetylation with ZnCl 2 , AcOH, and Ac 2 O in water (Scheme 2). 7,8,9 However, if ketone 3a is desired, it can be obtained from 3a·H 2 O by simply dissolving 3a·H 2 O in solvents such as EtOAc and stirring with Na 2 SO 4 overnight at rt, followed by filtration and concentration or by passing 3a·H 2 O through a short column of SiO 2 . In CDCl 3 , the hydrate gradually converted into the ketone as judged by 1 H NMR (25% ketone at 10 min, 50% ketone at 30 min, 67% ketone at 1 h, 90% ketone at 2 h, 100% ketone at 11 h). In CD 3 CN-D 2 O (1.5:1, v/v), ca. 21% of the ketone was formed from the hydrate at 1 h (ca. 22% at 7 h). When the ketone was subjected to the same solvent mixture, ca. 26% of the ketone remained at 1 h (ca. 25% at 7 h). It appears that a similar amount of the ketone was present at around 1 h regardless of whether the ketone or its hydrate was used. Similar conversions and ee's were also obtained for the e...
Dirhodium caprolactamate [Rh 2 (cap) 4 ] is a highly effective catalyst for the oxidative Mannich reaction. The reaction proceeds via C-H oxidation of a tertiary amine followed by nucleophilic capture. This green transformation is conducted in protic solvent using inexpensive T-HYDRO ® (70% t-BuOOH in water). Synthetically valuable γ-aminoalkyl-butenolides are obtained.The Mannich reaction remains a fundamentally important carbon-carbon bond forming reaction in organic synthesis. 1 Access to valuable Mannich addition products is made possible via reactive iminium ions that undergo capture with a rich assortment of nucleophiles. However, despite its well-documented scope and utility, methods for the formation of iminium ions remain limited. Strategies such as condensation or α-fragmentation often require harsh
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