Urethane cyclization

Several examples of transition metal catalysis for the synthesis of piperidines appeared this year. Palladium catalyzed intramolecular urethane cyclization onto an unactivated allylic alcohol was described as the key step in the stereoselective synthesis of the azasugar 1-deoxymannojirimycin . A new synthetic entry into the 2-azabicyclo[3.3.1]nonane framework was accomplished through a palladium mediated intramolecular coupling of amine tethered vinyl halides and ketone enolates in moderate yields . A palladium catalyzed decarboxylative carbonylation of 5-vinyl... 

Carbamate esters also produced 2-oxo compounds, e.g., the esters of 3-aminopropanol88 and of 3-halogenopropanol.87 A few compounds have thus been prepared from carbamates, e.g., 20, with an aromatic substituent88 [Eq. (10)] or a heterocyclic one.89 Ethyl 7V-(3-hydroxy-propyl) urethanes cyclize to 2-oxo compounds with sodium methoxide.78 An interesting novel approach was to react an A-(chloromethyl)-carbamate with olefins to yield 2190 [Eq. (11)]. 

In contrast to the above additions A-allyl- and substituted A-allyl-amides, -urethanes, -ureas and -thioureas undergo intramolecular cyclization only in 6(3-96% sulfuric acid to give the corresponding oxazolinium and thiazolinium salts. Treatment of these cations with base yields 2-oxazolines and 2-thiazolines in moderate to good yields. The reaction is illustrated by the conversion of A-2-phenylallylacetamide (342) into 2,5-dimethyl-5-phenyl-2-oxazoline (343) in 70% yield 70JOC3768) (see also Chapter 4.19). 

Sodium borohydride reduction of 4-substituted isoquinolinium salts led to vinylogous cyanamides, ureas, and urethanes, as well as the corresponding tetrahydroquinolines (640). Hydrogenation of /8-acylpyridinium salts (641) to vinylogous ureas was exploited in syntheses of alkaloids (642), leading, for instance, to lupinine, epilupinine, and corynantheidine (643, 644). Similarly, syntheses of dasycarpidone and epidasycarpidone were achieved (645) through isomerization of an a,/0-unsaturated 2-acylindole and cyclization of the resultant enamine. 

Reaction of aniline derivatives with 4-chlorobutyroyl chloride followed by cyclization with sodium ethoxide and subsequent thionation promoted by sonication gave the corresponding A -arylpyrrolidine-2-thiones 126. Zinc-mediated condensation of diethyl bromomalonate with 126 using iodine as activator gave the vinylogous urethanes 127 whose cyclization with PPA gave the tricyclic compound 128 which upon hydrolysis afforded the acid 129 (96TL9403). 

Benzaldehyde dimethyl acetal 121 reacts, for example, with the silylated allylic alcohol 645, in the presence of SnCl2-MeCOCl, via an intermediate analogous to 641, to the 3-methylenetetrahydrofuran 646 and methoxytrimethylsilane 13 a [182], whereas benzaldehyde dimethyl acetal 121 reacts with the silylated homoallylalco-hol 640 in the presence of TMSOTf 20 to afford exclusively the ds 4-vinyltetrahy-drofuran 647 and 13 a [183]. A related cyclization of an a-acetoxy urethane 648 containing an allyltrimethylsilane moiety gives the 3-vinylpyrrohdine 649 in 88% yield and trimethylsilyl acetate 142 [184, 185]. Likewise, methyl 2-formylamido-2-trimethylsilyloxypropionate reacts with allyltrimethylsilane 82 or other allyltri-methylsilanes to give methyl 2-formamido-2-aUyl-propionate and some d -unsatu-rated amino acid esters and HMDSO 7 [186] (Scheme 5.56). 

Further attempts to effect a one-step synthesis of the hasubanan skeleton via acid-catalyzed cyclization of urethane 28 and unsaturated amides 31 and 32 were explored, using trifluoroacetic acid (TFA) (Scheme 1). Treatment of... 

There have been reports that urethane was produced when the mixed-anhydride method was employed for the coupling of segments. However, studies on urethane formation during the aminolysis of mixed anhydrides of peptides have never been carried out. The anhydrides are too unstable to be isolated. The activated moiety of the peptide cyclizes too quickly to the 2,4-dialkyl-5(4//)-oxazolonc (see Section 2.23), and since the time allowed to generate the anhydride in segment couplings is always limited to avoid epimerization, one cannot exclude the possibility that the urethane that was produced originated by aminolysis of unconsumed chloroformate. 

FIGURE 7.34 Decomposition of the symmetrical anhydride of A-methoxycarbonyl-valine (R1 = CH3) in basic media.2 (A) The anhydride is in equilibrium with the acid anion and the 2-alkoxy-5(4//)-oxazolone. (B) The anhydride undergoes intramolecular acyl transfer to the urethane nitrogen, producing thelV.AT-fcwmethoxycarbonyldipeptide. (A) and (B) are initiated by proton abstraction. Double insertion of glycine can be explained by aminolysis of the AA -diprotected peptide that is activated by conversion to anhydride Moc-Gly-(Moc)Gly-0-Gly-Moc by reaction with the oxazolone. (C) The A,A -diacylated peptide eventually cyclizes to the IV.AT-disubstituted hydantoin as it ejects methoxy anion or (D) releases methoxycarbonyl from the peptide bond leading to formation of the -substituted dipeptide ester. 

An example of an intriguing ring-size dependency of the reaction pathway is that of the cyclization of vinylogous urethanes 328a-c with cinnamaldehyde (08T883). Pyrrolidine- and azepane-based esters 328a and c, respectively, give via aza-[3+3] annulation the expected... 

On treatment with benzeneselenenyl chloride two olefinic urethanes (214 and 217) underwent cyclization to afford piperidine derivatives (215 and 218, respectively) having the cis stereochemistry. Their reduction with triphenyltin hydride gave the same product (216). Removal of the blocking group from the nitrogen gave ( )-isosolenopsin A (Ic) (Scheme 6) (392). 

Amidoximes (96) react at the hydroxy group with isocyanates or carbamoyl chlorides. The resulting urethanes (105) can be cyclized in boiling acetic anhydride (Scheme 43). A,A-Disubstituted urethanes (106) as well as the A-carbamoyl compounds (108), give oxadiazolinones (107) (Schemes 43 and 44) <63HCA1073>. 

A strategy involving the intramolecular cyclization of acyclic urethanes as a key step has been widely employed since the first synthesis of this class of compounds. ... 

Lead tetraacetate oxidative cyclization of the A-(l-naphthylvinyl)urethane 30, derived from I -acetonaphthone 28, yields 4-naphthyl-2(3//)-oxazolone 32 as the... 

Deprotection of urethane-protected tertiary (3-hydroxy amides 37 with trifluoro-acetic acid followed by spontaneous cyclization of the liberated hydroxy amide affords oxazolines 38. The rate of deprotection is further accelerated by the addition of CaCl2. Examples are shown in Table 8.6 (Fig. 8.4 Scheme 8.15). 

Cyclization of pentamethylene bis-urethane (196) with formaldehyde gives a very low yield of the 1,3-diazocane (197), in contrast to the quite efficient preparation of the tetrazocane by condensation of the tetraaza chain (74JHC937). 

---