A New Synthesis of Benzylidene Acetals

Abstract

Aryl-halo-diazirines react under basic conditions with 1,3-cis-, 1,2-cis-, and 1,2-trans-diols to give acetals. Yields are high. Diastereoselectivities depend upon the diol and upon the reaction conditions. Thus, reaction of the 1,3-ris-diol 1 (Scheme 1 ) with 2 gave 3 as a single diastereoisomer. The 1,2-cis-diols 4 and 7 led to the endo-and exo-acetals 5/6 (93:7) and 8/9 (ca. lO:l), respectively. The 1,2-trans-diol 10, 16, and 19 reacted with 2 to afford 11/12 (90:10), 17/18 ( l : l ) , and 20jZl (6:1), respectively. Reaction of the (4-nitropheny1)diazirine 13 with 10 at higher temperatures yielded 14/15 (6:4). The uracil moiety, the acetamido group, and the enol-ether moiety are compatible with the reaction conditions. The diastereoselectivity is rationalized on the basis of a reaction sequence involving alkoxy-halogen exchange, which is regioselective or not, thermolysis of the ensuing alkoxydiazirine(s), protonation of the alkoxycarbene to form an (E)-configurated oxycarbenium ion, and attack of the neighboring oxy or hydroxy group, which is only possible for a limited range of conformers.Introduction. -The transformation into benzylidene acetals is one of the most reliable and useful ways to protect 1,3-and 1,2-diols [l] and is usually performed by acetalation, trans-acetalation, or reaction with (dihalomethy1)benzene [2]'). The first two methods require acid catalysis and are often high yielding. The third method requires base catalysis and proceeds in lower yields, especially for diols with labile groups such as the 0-(triphenylmethyl) substituent or for 1,2-trans-diols. Under acidic conditions, 2-phenyl-l,3-dioxanes are formed with a very high diastereoselectivity, while 2-phenyl-1,3-dioxolanes, as a rule, are obtained as mixtures of endo-and exo-isomers. This is not only important with regard to obtain pure products, but also with hindsight to further transformations of benzylidene acetals [lc] [4].A high-yielding method, using neutral or basic conditions for the diastereoselective preparation of 2-phenyl-1,3-dioxolanes and -dioxanes would be welcome, particularly for the selective transformation of acid-sensitive compounds, such as glycals. We have conceived of such a method, which consists in treating a diol with an aryl-halo-diazirine [5] under basic conditions, and which demonstrates the intramolecular insertion of alkoxyaryl carbenes into 0-H bonds 161. Halodiazirines are easily prepared by Graham's method from amidinium salts [7] [gal and undergo an alkoxide-halide exchange with alcohols to form alkoxy-aryl-diazirines [sa], which are thermally labile, loose N, in situ and form alkoxyaryl carbenes [gb]. For diols, insertion of these carbenes into the 0-H bond of a neighboring OH group should yield benzylidene acetals.

Glycosylidene Carbenes. Part 13. Synthesis and thermolysis of representative 1‐azi‐glycoses

“…The mixture was stirred for 30 min at -40", allowed to warm to r.t., diluted with CH2C12 (50 ml), and worked up as described above, to yield 21/22 9 : l (1 1.57 g) which crystallized during FC (250 g, CH2C12-CH2C12/MeOH 99 :1 +CH2CI2/MeOH 9 :2). The crystalline material (10.61 g, 96%) was recrystallized in CH2CI2/hexane to afford 21 (9 .10 g, 92%). b) Similarly, oxidation of 20 (2 g) with NCS (595 mg, 1.1 equiv.)…”

Synthesis and Absolute Configuration of Natural 2‐Pyrones

“…The method failed for the synthesis of (ent)-phomapyrone B that was thus synthesized via a different route, resulting in an assignment of the absolute configuration of natural phomapyrone B. 3146 showed the opposite sign as the natural product ([α] D 20 = -20, MeOH, c 0.30), [14] allowing the assignment of the absolute configuration of (R)-(-)-prolipyrone A for the compound from F. proliferatum. Also in this case some of 13 C-NMR signals of 6 slightly deviated from the reported data (Table S2).…”

“…Also in this case some of 13 C-NMR signals of 6 slightly deviated from the reported data (Table S2). [14] Although the methylated derivative 27 is not known to literature, a synthesis was performed to allow for future comparison of NMR spectroscopic data. Scheme 4.…”

Molecular Imprinted Based Quartz Crystal Microbalance Nanosensors for Mercury Detection

“…Because of this, there is a severe environmental and public health problem, and it has become necessary to detect it in the aqueous environment. [16][17][18][19] recommended method. For repeatability, four samples from the same concentration were prepared and measured.…”