Efficient access to functionalised medium-ring systems by radical fragmentation/radical addition to α-iodoketones

Functionalised medium-ring systems of various sizes can be efficiently prepared by a novel approach that embodies a radical-induced fragmentation of bicyclic β-hydroxy ketones, followed by a second radical-coupling reaction. The unexpected reactivity of α-keto radicals is also discussed.     

Synthesis and antimalarial activity of new 1,2,4,5-tetroxanes and novel alkoxy-substituted 1,2,4,5-tetroxanes derived from primary gem-dihydroperoxides

A convenient synthesis of new unsymmetrically substituted 1,2,4,5-tetroxanes and novel unsymmetrical alkoxy-substituted 1,2,4,5-tetroxanes starting from primary dihydroperoxides was developed. The structure of some tetroxanes was unambiguously assigned by X-ray crystal analysis giving interesting insights in the configuration and conformation of disubstituted 1,2,4,5-tetroxanes. Some of these tetroxanes showed notable antimalarial activity in vitro.     

A new route to the synthesis of thieno[2,3-d]pyrimidin-4（3H）-one derivatives catalyzed by 12-tungstophosphoric acid （H3PW12040）

Heteropoly acid H3PW12040 （PW） has been used as an effective catalyst for the synthesis of thieno[2,3-d]pyrimidin-4（3H）-one derivatives. The present methodology offers several advantages, such as high yields, short reaction times, mild reaction condition and a recyclable catalyst with a very easy work up.     

A selective and convenient ruthenium mediated method for the synthesis of mixed acetals and orthoesters

Addition of alcohols and phenols to O-allyl compounds (allyl ethers and acrolein acetals), catalyzed by [RuCl₂(PPh₃)₃], was examined. Intramolecular addition of an OH group, leading to the formation of cyclic acetals and orthoesters, was also investigated. As a result, a new, selective and convenient method for the synthesis of symmetrical and unsymmetrical (mixed) acetals and orthoesters was developed.     

Formation of Orthoesters of Oleanolic Acid During Königs-Knorr Glycosidations

Summary.  The formation of orthoesters during K?nigs-Knorr reactions is described. Diphenylmethyl oleanolate reacts with 1 → 4 linked disaccharide donors to orthoesters instead of the expected glycosides. The reaction with acetobromoglucose gave a mixture of orthoester and glycoside. The influence of the structure of the glycosyl donors and of the reaction conditions on the formation of orthoesters is discussed. Received February 12, 2001. Accepted February 22, 2001     

Tungstate sulfuric acid as an efficient catalyst for the synthesis of benzoxazoles and benzothiazoles under solvent-free conditions

Tungstate sulfuric acid (TSA) has been found to be an efficient and reusable catalyst for the synthesis of benzoxazole and benzothiazole derivatives via reactions of orthoesters with o-aminophenols or o-aminothiophenols in high yields.     

C−C Bond Formation between Fischer Carbene Complexes and Allylic Alcohols by a [3,4] Sigmatropic Rearrangement Promoted by a [1,2] M(CO)5 Shift

A [1,2] M(CO) ₅ shift promotes a [3,4] sigmatropic rearrangement after the addition of allylic alcohols to Fischer alkenylcarbene complexes of tungsten or chromium in the presence of alkoxide ions. This opens a new synthetic route to the adducts 1 . The reaction is also applicable to propargylic alcohols. [M]=Mo(CO)₅, W(CO)₅.     

New Phragmalin‐Type Limonoid Orthoesters from the Bark of Chukrasia tabularis var. velutina

Three new phragmalin‐type limonoid orthoesters, velutabularins K–M ( 1 – 3 ) and seven structurally related known compounds were isolated from the bark of Chukrasia tabularis var. velutina. The structures of these compounds were elucidated by spectroscopic analysis. The structure of 1 was confirmed by single crystal X‐ray diffraction. In addition, the brine shrimp lethality of compounds 1, 3 , and the known compounds was evaluated.     

2-DICHLOROMETHYL-1,3-DIOXOLAN-2-YL ORTHOESTERS. A POTENTIAL PROTECTING GROUP FOR SUGAR DERIVATIVES

Reactions of 2-dichloromethylene-1,3-dioxolane (2) with acetal protected sugar derivatives in neutral media gave 2-dichloromethyl-1,3-dioxolan-2-yl orthoesters. These orthoesters are readily hydrolysed under mild acidic conditions offering a new method for the preparation of readily removable protecting groups. This strategy was realised in the preparation of 6-O-acetyl-3,5-di-O-methyl-1,2-O-isopropylidene-α-d-glucofuranose.     

Comparison of Several Glucuronate Glycosyl Donors

Methyl 3,4‐di‐O‐benzyl‐[(S)‐1,2‐O‐(1‐cyanoethylidene)]‐α‐d‐glucopyranuronate (12), methyl 3,4‐di‐O‐benzyl‐[(S)‐1,2‐O‐(1‐ethoxyethylidene)]‐α‐d‐glucopyranuronate (14), methyl 2‐O‐acetyl‐3,4‐di‐O‐benzyl‐α‐d‐glucopyranuronate bromide (15), methyl (2‐O‐acetyl‐3,4‐di‐O‐benzyl‐α‐d‐glucopyranosyl)uronate trichloroacetimidate (17), and methyl (2,3,4‐tri‐O‐benzyl‐α/β‐d‐glucopyranosyl)uronate trichloroacetimidate (30) were synthesized and used as glycosyl donors. Glycosylation reactions of 12 with (5‐R)‐2,3,4,5 ? tetrahydro‐5‐trityloxymethyl‐2‐furanone (32) and 14,15,17 with the corresponding (5‐R)‐2,3,4,5‐tetrahydro‐5‐hydroxymethyl‐2‐furanone (31) provided the exclusively β‐linked glucuronide 33 in 69%, 28%, 45%, and 71% yield, respectively. The coupling of donor 30 with acceptor 31 furnished the glucuronated lactone 35 in 70% yield with a surprisingly high content (20%) of the undesired α‐linked sugar residue. The structure of 33 was proved by NMR and X‐ray diffraction studies. In a model reaction a complete deprotection procedure of the glucuronic acid lactone conjugation was demonstrated.