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Carbapenam is a very important skeleton of beta-lactam antibiotics, and it has a highly strained structure. When enynes 9 were treated with RuH(2)CO(PPh(3))(3) (10 mol %) in toluene upon heating, carbapenams 10 were obtained in good yields.
[reaction: see text]. Addition of the configurationally stable organolithium species produced by enantioselective deprotonation of N-Boc-N-(p-methoxyphenyl) allylamines to alpha,beta-unsaturated carbonyl compounds affords 1,4-addition products in good yields with high diastereomeric and enantiomeric ratios. Further transformations of these compounds provide [3.3.0]-, [4.3.0]-, [5.3.0]-, and [5.4.0]-carbocycles and heterocycles with high stereoselectivities.
[reaction: see text]. Treatment of the allenyl sulfoxides and sulfones possessing a proper delta-hydroxy appendage at the C-1 position with potassium tert-butoxide effected endo mode ring closure at the sp-hybridized carbon center of the allenyl moiety to provide the five- to eight-membered oxacycles in high yields.
The potential of macrocyclic diterpenoids to afford natural product-like polycyclic compounds was demonstrated by the conversion of two lathyrane Euphorbia factors into a series of densely functionalized diterpenoids of unnatural skeletal type. Apparently, Nature is far from having fully exploited the built-in reactivity of these compounds to generate chemical diversity.
Intramolecular coupling reactions of ketene dithioacetal groups with enol ether and alcohol nucleophiles have been studied. The reactions were initiated by an anodic oxidation of the ketene dithioacetal and proved to be compatible with the formation of five- or six-member rings, as well as the stereoselective generation of quaternary carbons.
[reaction: see text] Photocycloaddition of 2-pyridones yields a rigid polycyclic product containing a 1,5-cyclooctadiene. The cis isomer, with the alkenes in close proximity, undergoes a transannular reaction when treated with chlorine to give a polyquinane product. The chlorination reaction involves migration of an amide nitrogen and forms a single isomer, generating eight stereogenic centers in two steps.
[reaction: see text] Suzuki coupling of 18 and 30 affords 9 as an interconverting mixture of two atropisomers. Treatment with LDA at -23 degrees C affords the macrocyclic ketone 8 in 57% yield.
[reaction: see text] Acyclic diastereoselection is achieved for the formation of thioaminyl acetals. The highly intramolecular stereocontrolled S(N)2 displacement of the thioaminyls allows for the formation of cyclic nucleoside derivatives. This versatile approach may provide easy access to a large variety of N-glycosides.
[reaction: see text] An approach to the macrocyclic core of apicularen A is described. Thus, cross-coupling of the aryl triflate 7 with the vinylstannane 19 provided the styrene 20. Deprotection led to the dihydroxy acid 22. Through a size-selective macrolactonization, the 12-membered macrolactone 23 was obtained. Treatment of 24 with N-phenyl selenophthalimide gave the desired trans-pyran 24. This approach might parallel the biosynthetic pathway.
[reaction: see text] The four components shown are assembled in two steps, and the product is converted to araneosene by a short sequence.
[reaction: see text] Enolizable vinyl quinone mono- and diimide substrates yield protected 6-hydroxy and 6-amino dihydroquinolines by thermal electrocyclization. Aromatization provides the corresponding quinolines in quantitative yields. The quinone monoimide substrates undergo clean photochemical conversion to 5-hydroxy indoles.
[reaction: see text] Ceric ammonium nitrate mediates the oxidative 5-endo radical-polar crossover reactions of beta-enamide esters to give 5,5-C,O-disubstituted-gamma-lactams. Trapping of the intermediate cations leads to 5-hydroxy- or 5-alkoxy-gamma-lactams depending upon the reaction conditions. The methodology was used to synthesize the basic heterocyclic ring fragments of the natural products L-755,807, Quinolacticin C, and PI-091.
A simple and effective synthesis of trans aryl-substituted piperazines using a Brønsted acid and manganese(0) is described. [reaction: see text]
[reaction: see text] Treatment of 1-iodomethyl-1,5-bis-epoxides with zinc in refluxing ethanol affords cis or trans 2-vinyl-5-(1-hydroxyethy)-substituted tetrahydrofurans stereospecifically in high yield.
[reaction: see text]. A short and efficient synthesis of the beta-carbolinone ring system was achieved using a rhodium(II)-catalyzed [3 + 2]-cycloaddition, a Pd(0)-catalyzed C-N amination reaction, and a subsequent intramolecular Heck reaction as the key synthetic steps.
[reaction: see text] The chemoselectivity of the ruthenium-catalyzed hydrative diyne cylization is explored in an expeditious synthesis of the tricyclic alkaloids cylindricine C, D, and E.
[reaction: see text]. The intramolecular vinylogous Morita-Baylis-Hillman reaction was explored to access the central cyclopentane ring of FR182877. The reaction manifold and product distribution is strikingly solvent and substrate dependent.
Quinolines substituted in the 3-position by an iodo or phenylseleno group are readily prepared in good to excellent yields by the reaction of propargylic anilines with appropriate electrophiles under mild reaction conditions. [reaction: see text]