Diastereoselective Photochemical Synthesis of α-Amino-β-hydroxyketones by Photocycloaddition of Carbonyl Compounds to 2,5-Dimethyl-4-isobutyloxazole

Summary. The photocycloaddition of aldehydes and α-ketoesters to 2,5-dimethyl-4-isobutyloxazole leads to bicyclic oxetanes with high to moderate (exo) diastereoselectivity that can be easily ring-opened to give α-amino-β-hydroxyketones.     

Synthesis of erythro-alpha-amino beta-hydroxy carboxylic acid esters by diastereoselective photocycloaddition of 5-methoxyoxazoles with aldehydes

A new photoaldol route to alpha-amino-beta-hydroxy carboxylic acid esters is initiated by the photocycloaddition of aromatic or aliphatic aldehydes to 5-methoxyoxazoles. The 4-unsubstituted 5-methyloxazole 1 gave the cycloadducts 8a-f in high yields and excellent exo-diastereoselectivities. Hydrolysis of 8a-f gives the N-acetyl alpha-amino-beta-hydroxy esters 9a-f, which could be subsequently converted into the corresponding Z-didehydro alpha-amino acids 10a-f. Quartenary alpha-amino-beta-hydroxy esters 12, 14, 16, 18, and 20, which are stable against dehydration, were obtained from the 4-alkylated 5-methoxyoxazoles 2-6, in most cases highly erythro-selective due to the high degree of stereocontrol (exo) at the photocycloaddition (to give 11, 13, 15, 17, and 19) level. The relative configurations of the N-acetyl alpha-amino-beta-hydroxy esters were determined by NMR spectroscopy and comparison with chiral pool-derived compounds as well as by X-ray structure determination of the ester 23, formed by hydrolysis of the cycloadduct 22, derived from photocycloaddition of propionaldehyde to the isoleucine-derived oxazole 21.     

Convenient synthesis and antitumor evaluation of some new 9-ethyl-3-(hetaryl)carbazoles

Abstract

In continuing our efforts to develop new potent anticancer candidates, a new series of 9-ethylcarbazoles carrying at position 3 various heterocyclic substituents such as 2-imino-2H-chromenes 5a–e, 2-oxo-2H-chromenes 6a–e, 3-imino-3H-benzo[f]chromene 8, 3-oxo-3H-benzo[f]chromene 9, 2-pyridones 11, 14, pyrazole 19, pyrimidine 23, pyrido[1,2-a]pyrimidine 27, 2H-pyran-2-one 30, and pyrano[2,3-d]pyrimidinetrione 34 were efficiently synthesized, characterized and evaluated for their in vitro antitumor activity. The mechanism for the synthesis of compounds was also discussed. Most of the synthesized compounds were displayed the considerable anticancer activities against three human tumor cells lines, in particular, colon carcinoma (HCT-116), hepatocellular carcinoma (HepG-2) and breast cancer (MCF-7). Compound 6d proves as most active molecule in this study with special effectiveness against the human HCT-116 and HepG-2 as its IC₅₀ values are 1.50, 0.90?μM, respectively, when doxorubicin is compared. Compound 34 was also found to have high activity against HepG-2, HCT-116 and moderate activity against MCF-7.     

Synthesis of Some New 1,2,3,4‐Tetrahydropyrimidine‐2‐thione and Their Thiazolo[3,2‐a]pyrimidine,Thiazino and Benzothiazepine Derivatives

The starting N‐(2‐pyridyl)‐6‐methyl‐4‐phenyl‐2‐thioxo‐1,2,3,4‐tetrahydropyrimidine‐5‐carboxamide ( 4 ) was used as a key intermediate for the synthesis of new 1,2,3,4‐tetrahydropyrimidine‐2‐thione and their thiazolo[3,2‐a]pyrimidine, thiazino and benzothiazipen derivatives. The reaction of 4 with haloketones in ethanol catalyzed by base afforded the corresponding thiophenopyrimidine and pyrimidothiazipine derivatives 5 , 6 , 7 , 8 , 9 , 10 . Methylation and formylation of 4 led to the pyrimidine derivatives 15 and 16 , respectively. The preventative compounds were established on the basis of elemental and spectral data.     

Eco-friendly solvent-free synthesis of thiazolylpyrazole derivatives

Highly versatile ethyl 3-thiosemicarbazidobutanoate was ball-milled with phenacyl bromide to afford the corresponding ethyl 3-[(4-phenyl-2-thiazolyl)hydrazono]butanoate which underwent heterocyclization by heating in ethanolic sodium acetate to give thiazolylpyrazolone that coupled chemoselectively with aromatic diazonium salts to furnish arylhydrazonothiazolylpyrazoles. Vilsmeier-Haack reaction of ethyl 3-thiosemicarbazidobutanoate furnished selectively thiazolylpyrazole. A series of pyrazolylthiosemicarbazones were synthesized by solid-state technique which allowed waste-free production. The reaction of pyrazolylthiosemicarbazones with phenacyl bromide afforded the corresponding 2-(arylidenehydrazino)-4-phenylthiazoles in quantitative yields. Correspondence: Samir Bondock, Department of Chemistry, Faculty of Science, Mansoura University, ET-31556 Mansoura, Egypt.     

Stereoselective generation of vicinal stereogenic quaternary centers by photocycloaddition of 5-methoxy oxazoles to alpha-keto esters: synthesis of erythro beta-hydroxy dimethyl aspartates

The photocycloaddition of methyl pyruvate and methyl phenylglyoxylate, respectively, to 5-methoxy oxazoles bearing additional substituents at C-2 and C-4 leads to bicyclic oxetanes 2 and 3 with high to moderate (exo) diastereoselectivity that can be easily ring-opened to give bis-quaternary aspartic acid diester derivatives 4 and 5.     

Diastereoselective Photochemical Synthesis of α-Amino-β-hydroxyketones by Photocycloaddition of Carbonyl Compounds to 2,5-Dimethyl-4-isobutyloxazole

The photocycloaddition of aldehydes and α-ketoesters to 2,5-dimethyl-4-isobutyloxazole leads to bicyclic oxetanes with high to moderate (exo) diastereoselectivity that can be easily ring-opened to give α-amino-β-hydroxyketones.     

One‐pot synthesis of pyridine derivatives via diels‐alder reactions of 2,4‐dimethyl‐5‐methoxyoxazole

A novel series of pyridine derivatives with anticipated biological activity have been synthesized via Diels‐Alder reactions of 2,4‐dimethyl‐5‐methoxyoxazole with different types of dienophiles. The regioselectivity of the cycloaddition was inverted from methylacrylate to tert‐butylacrylate. The structural elucidation of the new compounds was carried on the basis of spectral and X‐ray analyses. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:49–55, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20064