Dehydro derivatives

A steroid very closely related structurally to cholesterol is its 7 dehydro derivative 7 Dehydrocholesterol is formed by enzymatic oxidation of cholesterol and has a conju gated diene unit m its B ring 7 Dehydrocholesterol is present m the tissues of the skin where it is transformed to vitamin D3 by a sunlight induced photochemical reaction... 

Whereas 2-deoxy-fortimicin A has full antibacterial activity (205), and the 2-deoxy-3-demethoxy- derivative is even more active than fortimicin A (236), the 2,5-dideoxy derivative has poor activity, as do the 2-deoxy-2-chloro and the 2,5-dideoxy-4,5-dehydro derivatives. The 2-amino-3-0-demethyl-2-deoxy analogue has quite good activity (229). 

In -isosparteine (47) both tertiary protons at C-6 and C-11 are arranged in the CIS position to the free electron pair on nitrogen as indicated by the NMR spectrum (80). Much earlier X-ray analysis showed that all rings in crystalline a-isospa teine (46) are present in stable chair conformations (SJ). A comparative rate (extrapolated) at 65 for a-isosparteine (5.0) and sparteine (1.0) has been calculated (82). Also it has been reported that (8-isosparteine gave the dehydro derivative under mild conditions and the didehydro under more drastic conditions (times, temperatures not given) (60). 

It was assumed that pyridine derivative 181 yielded pyrido[l,2-c]pyrimidine betaine 182 under catalytic hydrogenation conditions over (5,S)-Et-DuPHoS-Ph catalyst (99TL1211). 6,7-Dehydro derivative 184 of trequinsin (3) was obtained from pyrimidinone 183 by heating in an 1 1 mixture of MeOH and cone. HCl under reflux (97IJC(B)349). 

Annulene and dehydro[22]annulene are also diatropic. A dehydroben-zo[22]annulene has been prepared that has eight C=C units, is planar and possesses a weak induced ring current. In the latter compound there are 13 outer protons at 6.25-8.45 8 and 7 inner protons at 0.70-3.45 5. Some aromatic bridged [22]annu-lenes are also known. The [26]annulene has not yet been prepared, but several dehydro[26]annulenes are aromatic.Furthermore, the dianion of 1,3,7,9,13,15, 19,21-octadehydro[24]annulene is another 26-electron system that is aromatic. Ojima and co-workers prepared bridged dehydro derivatives of [26], [30], and [34]annulenes. All of these are diatropic. The same workers prepared a bridged tetradehydro[38]annulene, which showed no ring current. On the other hand, the dianion of the cyclophane 89 also has 38 perimeter electrons, and this species is diatropic. ... 

Alkaloids, e.g. reserpine, rescinnamine 105 C, 2 h Induced fluorescence (Xn > 500 nm, cut off filter). Possibly formation of 3-dehydro derivatives. [19]... 

Treatment of a 1,2-dehydro derivative (1 mmol) with an equimolar amount of 0,0-diethyl S-hydrogen phosphorodithioate (1 mmol) in benzene (3 ml) for 24 h quantitatively yielded a crude anomeric mixture of the title compounds after evaporation in... 

Fig. 2.50. Profiles of calendula (a), milk-thistle (b) and passion flower (c) tinctures (each 60 per cent v/v, ethanol) with the HPLC-MS attributions of the components detected, lc = quercetin-3-O-ruti-nosylrhamnoside 2c = rutin 3c = isorhamnetin-3-O-ruti-nosylrhamnoside 4c = isoquercitrin 5c = isorhamnetin-3-O-gluco-sylglucoside 6c = narcissin 7c = isorhamnetin-3-O-glucoside lm = taxifolin 2m = siliydianin 3m = silychristin 4m = oxy derivative of silybin/isosilybin isomers 5m and 6m = 2,3-dehydro derivatives of silybin/isosilybin isomers 7m = silybin 8m = isosilybin lp = 6,8-diC-glucosylapigenin 2p = isoschaftoside 3p = shaftoside 4p = homoorientin 5p = isovetexin-2"-0-glucoside 6p = vitexin. Reprinted with permission from A. R. Biha et al. [150].

At elevated temperatures, extremes in pH, and/or moisture, the dipeptide sweetner aspartame is converted into cyclo(Asp-Phe). The dehydro derivatives of cyclo(Phe-Phe) were found to inhibit cell division, while cyclo(Asp-Pro) caused inhibition of dietary fat intake. ... 

The intramolecular thermal cyclotrimerization of dodeca-1,6,11-triyne (110) at 450-600 °C afforded l,2,3,6,7,8-hexahydro[a5]indacene (112) and dehydro derivatives. An exothermic cycloaromatization mechanism has been proposed. An initial formation of a single bond gives diradical (111) which is then trapped by an alkyne. °... 

Ninomiya et al. (38) synthesized dibenzopyrrocolines by enamine pho-tocyclization. In model studies designed to determine the feasibility of the photochemical reaction, an ether solution of the enamine 44a was irradiated at room temperature, and tetrahydrodibenzopyrrocoline 45a and a dehydro derivative (46) were obtained in 10 and 20% yield, respectively. The more abundant compound 46 was reduced to 45a by tin and hydrochloric acid. 

A large number of cembrane diterpenoids have been obtained from marine organisms, particularly soft corals. 2-Hydroxynephthenol (116) was isolated from Litophyton viridis. A dehydro-derivative (117) of epoxynephthenol was... 

Reaction of various aldehydes with hydrogen sulfide leads to substituted thiophenes, dihydrothiophenes, dithiolanes and trithiolane, as well as to six-membered ring thiopyran derivatives and dithiins. Ledl (33) obtained 2,4-dimethylthiophene (1, R Me) as a product of the reaction of propionaldehyde with hydrogen sulfide in the presence of ammonia. Sultan (29) reported the formation of 2,4-diethylthiophene (1, R - Et), 2,4-dibutyl-thiophene (1, R - Bu), and their dehydro derivatives from the reaction of ammonium sulfide with butyraldehyde and caproaldehyde (hexanal), respectively. The mechanism suggested for their formation is depicted in Scheme 1. Space limitations do not allow us to discuss the mechanism here in detail (for additional information, see ref. 29). 

Treatment of this last intermediate with DDQ leads to the dehydrogenation of the lactam ring to form the 1-dehydro derivative dutasteride (46-5) [45]. 

A diastereoisomer of this compound, its 5-dehydro derivative (119), and two artifacts, 120 and the 4,5-acetonide, have been isolated from the... 

Reaction of the nitrile oxide (498) with the enone (499) gave after chromatography the expected isoxazoline (500) plus some of its dehydro derivative (76TL3983). Hydrogenation of (500) using 30% Pd/SrC03 as catalyst yielded an enaminoketone (501) which was further hydrolyzed to the diketone (502 Scheme 110). This compound is a gem-dimethyl isomer of dehydrocycloheximide, a key intermediate in a previously reported synthesis of the glutarimide antibiotic cycloheximide. 

Diphenylcarbazide, PhNHNHCONNHPh, or its dehydro derivative diphenylcarbazone, have long been known to form a water-soluble red-violet complex with chromium(VI) but this can be... 

A theoretical study of the geometries of several even more highly strained dehydro derivatives of la has appeared12. 

---