The structure of the methyl thioglycollate monoadduct 2 of the NCS chromophore, including the absolute stereochemistry, was. The presence of the cumulene-enyne intermediate and the rearrangement to a biradical were supported by data from low-temperature NMR investigations. We first deduced the proposed gross structure of a neocarzinostatin5 chromophore-methyl thioglycollate adduct and then extended this effort by determining the absolute stereochemistry of the adduct.
Since there is no stereochemical loss in the formation of the adduct, we also know the absolute stereochemistry of the NCS chromophore itself. In addition, a model study is presented which aims to avoid some of the problems associated with the intramolecular reaction described above. Each of the four diastereomers of model compounds ~ and 6 was synthesized in non-racemic form as outlined in Scheme II and separately converted to its (R)- and (S)-α-methoxy-α-(trifluoromethyl)phenylacetic acid ester (MTPA derivative , Mosher ester)11 with excess MTPA chloride and 4-(dimethylamino)pyridine (0.6 M each} in methylene chloride at 23°C for 1.5 h.
Quantitative determination of the data was achieved by calculating the x71l function11 and calculating the reasonable value of O.lO ppm for t1. Reaction of the antitumor antibiotic neocarzinostatin chromophore (I) with thiol 2 (0.2 M) in 0.5 M methanolacetic acid C} has been shown to produce the stable mono- and bisthiol adducts Sand 6, in the ratio I:I, respectively. 1 The sequence I - 3 - 4 - S (Scheme I) was suggested to be responsible for the formation of 5.2 In this pathway, biradical 4 is of particular importance because free radical intermediates are involved in the cleavage of DNA by thiol-activated 1.1 To gain further insight into the mechanistic details of the I + 2 - S + 6 transformation, we studied this reaction at low temperature. The remainder of the anion solution was added over a three-minute period, forty minutes after the initial injection; Heat to 0°C and stir for 10 minutes.
Storage of the compounds at -20° C frozen in benzene with 2,6-di-tert-butyl-4-methylphenol (BH1) added inhibited decomposition.
M . Product 6
The methanolic solution of the chromophore was concentrated in vacuo to give an orange-brown residue. The synthetic studies of neocarzinostatin chromophore presented here are an extension of work already done in Myers' laboratories. l Model compound 1, which lacks the oxygen functionality of ClO and Cll and has a pivaloate ester in place of the carbonate moiety, is the target. The epoxy aldehyde 5 was obtained by cleavage of the dimethyl acetal with hydrogen peroxide in methylene chloride/tert-butyl alcohol with trichloroacetic acid present.
The attempted cyclization of 6 led mainly to the reduced acetylene and a small reduction of the unsaturated system in the lower half of the molecule. The desired cyclized process will have significant strain energy, most of which will be manifested in the transition state. Diol LiHMDS, THF, -78° C Mainly recovered S.M.; two small bases added to isolated substrate products contain aldehyde.
Diol KHMDS, Toluene, -78° C Mainly recovered S.M.; minor isolated Substrate added to base product contains an aldehyde peak through. Bis-TMS KHMDS, Toluene, -78° C Mainly recovered S.M.; minor isolated Base added to substrate products contain aldehyde peaks. Bis-TMS LiTMP, THF, -78° C Mainly recovered S.M.; small products Substrate added to base contains aldehyde peaks; apparent.
Bis-TMS LiTMP, THF, 0° C Mostly recovered S.M.; some small bases added to substrate uncharacterized products; Visible THF. Bis-TMS LiTMP, Et20, -78° C Mostly recovered S.M.; The small isolated substrate added to the base products had no aldehyde peaks, but. Dimethylhydrazone 9 was prepared from 8 by reaction with 1,1-dimethylhydrazine in the presence of sodium sulfate.
Reaction of 8 with triflic anhydride in the presence of 2,6-di-ten-butyl-4-methylpyridine at -60°C afforded the enol triflate 10. In the third (Tius), extended enolate formation is possible and can play a role in reducing yield. The large amount of initial material recovered in the latter case (60%) compared to that of Danishefsky (18%) and Kende (30%) may indicate that such a process is occurring.
The first step of the synthetic route to tert-octyl-tert-butyl amine was reversed. When the mixture was warmed to room temperature, 1M Na 2 S 2 O 3 (10 mL) was added to quench excess iodine.
Hydrogen peroxide (70%, 0.5 ml) and trichloroacetic acid solution (130 µL of a 0.51 g/ml solution in methylene chloride) were then introduced. The cooling bath was removed, the flask was wrapped in foil and a blow shield was placed in front of the flask. The reaction mixture was poured into 10 ml of ice-cold brine solution and the flask was rinsed with 50% ethyl. The mixture was diluted with ethyl acetate (15 mL) and the layers were separated after vigorous stirring.
An argon balloon was attached to the flask and then bis(trimethylsilyl)acetamide (1 mL, 4.0 mmol, 148 equiv) was added. After cooling to room temperature, the reaction mixture was diluted with 15% ethyl acetate/hexanes (15 mL) and then washed with water (10 mL). Polar impurities were removed by flash chromatography (10% ethyl acetate/hexanes) through a short plug of silica to afford a clear oil (13.1 mg, 82%).
The mixture was diluted with 75% ethyl acetate/hexanes (E/H, 50 mL) and the layers were separated after stirring in a separatory funnel. The organics were dried (Na-2SO4) and concentrated in vacuo to give a crude orange-brown oil. The flask was fitted with an argon balloon and then bis(tri-methylsilyl)acetamide (2 mL, 8.09 mmol, 22.8 equiv) was added via syringe.
The reaction mixture was diluted with 15% ethyl acetate/hexanes (E/H, 25 mL) and then washed rapidly with water to avoid hydrolysis of the trimethylsilyl ether. Concentration in vacuo produced a light yellow oil, which was determined to be a 60:40 mixture of starting material:product by NMR. The crude product mixture was re-submitted to the reaction conditions except at ambient temperature.
Concentration in vacuo gave a yellow oil which was purified by flash chromatography (15% FJH) to give a pale yellow oil (26.2 mg, 80% yield). The aldehyde substrate (30 mg, 64.8 Jmol, 1 equiv) was added as a solution in toluene and then concentrated in vacuo. The reaction mixture was diluted with 20% ethyl acetate/hexanes (EIH, 15 mL) and washed with water (10 mL).
After coming to room temperature, the reaction mixture was diluted with 30% ethyl acetate/hexanes (E/H, 15 mL) and then washed with half-saturated brine (20 mL).
