4-penten-l-yloxy group attached to C-l of each. On attack by an electrophile, this group snakes back on itself to form an intermediate tetrahydrofuran oxonium salt. This in termediate expels the tetrahydrofuran to yield a second intermediate oxonium salt, unsaturated between the C-l and the pyran oxygen. Fi nally, this intermediate reacts with a free hydroxyl group of a second glycoside to join the two. The electrophilic reagent can be either N-bromosuccinimide or a collidine complex of iodonium perchlorate. The tetrahydrofuran ex pelled is then either the 2-bromomethyl or 2-iodomethyl compound. The second glycoside may also have a pentenyloxy group at C-l, setting the stage for coupling with a third. But with two pentyloxyglycosides in the reaction mixture, the chemists had to ensure that the right one would react, saving the other for later coupling. This they accomplished by what Fraser-Reid calls "arming" one and "disarming" the other. Protecting the hydroxyl at C-2 as an electrondonating ether permitted formation of the tetrahydrofuran oxonium in termediate, and the reaction pro ceeded. Putting an electron-with drawing ester group at C-2 inhibited this step and prevented that glyco side from coupling. Thus, ethers were "armed" glycosides, and es ters were "disarmed" ones. In coupling of two glycosides, the configuration at C-l may be either a (the oxygen points "down") or β (the oxygen is "up"). The Durham workers found that they could of ten heavily favor one configuration over the other by the choice of re action solvent. The Duke team then marched through the synthesis of the penta saccharide. First they coupled a benzylated glucosamine to one free hy droxyl of an acetylated inositol. Then they attached three protected mannose residues in a row, each one at a different type of hydroxyl. In each of these couplings, they had to ensure forming the «-configuration at C-l. The signal-transducing glycopro tein of the trypanosome has a pro tein coupled through a 2-aminoethyl phosphate group to the terminal mannose of the pentasaccharide.
This portion of the molecule is embedded in the cell membrane. There is a pattern of four galactoses bound to a central mannose. And the inositol end is linked to a phosphati-
dyl (diglyceride phosphate) group, This group is farthest out on the cell surface and may be the portion that interacts with hormones, Stephen Stinson
Vietnam seeks to end scientific isolation Vietnam has long been isolated from Western countries through a com bination of its own closed-door pol icies and an embargo applied by the U.S. and other Western govern ments. Thus, unusual interest was aroused by the visit to the U.S. last month of two top Vietnamese sci entists (C&EN, Oct. 16, page 8). Physicist Nguyen Van Hieu, 51, president of Vietnam's National Centre for Scientific Research, based in Hanoi, and chemist Ho Si Thoang, 52, vice president of the center and head of its branch in Ho Chi Minh City (formerly Saigon), made an in tensive round of visits across the country, seeking to build ties with U.S. scientists and institutions. In
deed, they stressed in a visit to the American Chemical Society in Wash ington, D.C., and other contacts, that Vietnam was "very closed to the West" in the past, but now wants "to send [its] students and scientists abroad, and invite many scientists from abroad to visit." In their discussions and in an in terview w i t h C&EN, Hieu a n d Thoang also gave a rare view of the priorities, prospects, and problems of science in Vietnam. "Vietnam's science is very young," Hieu points out. Before 1954, when North Vietnam won its indepen dence from France, there were few scientists, but they included several eminent, French-trained figures. In
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Scientists Thoang (left) and Hieu of Vietnam confer with ACS'sJohn Malin its first few years, the communist government founded medical and teachers colleges, schools for science, technology, agriculture, forestry, and fisheries, and a national research organization. And it began to send students to study in the Soviet Union and East Europe. Hieu and Thoang both earned their doctorates in the U.S.S.R., for example. The outbreak of large-scale fighting against U.S. and South Vietnamese forces in 1964, and U.S. bombing of North Vietnam, put a damper on scientific activities. However, Hieu notes, Hanoi continued to send students to the socialist countries for study—"knowing it would need scientists and engineers for reconstruction of the country after the w a r / ' The National Centre for Scientific Research was formed in 1975, after the war ended earlier that year with communist occupation of Saigon. Following the Soviet model of a centralized research organization, the center supervises the work of 40 research institutes employing some 2000 scientists. Hanoi has research institutes of chemistry, polymer chemistry, and the chemistry of natural products, with a chemical engineering institute under discussion. Ho Chi Minh City has institutes of chemistry and chemical engineering. These institutes do postgraduate training as well as research. University labs also carry out chemical research, as do institutes run by government industrial ministries. The research institutes are very small, Thoang adds, with 30 to 70 scientists each. In all, he says, there are about 500 Ph.D. chemists in Vietnam, a p p r o x i m a t e l y 100 of
them at national center laboratories. Vietnam, with a population estimated at more than 66 million, is one of the world's poorest countries, with an estimated per capita income of $180 in 1987. It is heavily dependent on foreign aid, mainly from socialist countries. Hieu and Thoang frankly describe the resulting low level of their nation's scientific establishment—poor in equipment, facilities, supplies of research chemicals, and funding. Obtaining scientific books and journals is a major problem. As a result, "we can't work much on basic scientific research. We must work mostly on applied topics," funded by contracts with industrial enterprises and government ministries, Hieu notes. In the chemical area, this includes research on natural products, such as perfume ingredients, medicinal plants, and raw rubber. Vietnam also has significant deposits of rare earths, other minerals, and offshore oil, all of which are under study in chemical labs. The lack of facilities has relatively little effect on theoreticians like Hieu. Indeed, says Judith L. Ladinsky of the University of Wisconsin, chairman of the U.S. Committee for Scientific Cooperation with Vietnam, Vietnamese scientists do comparable work to that in the West in areas of theoretical physics and mathematics requiring little more than paper and pen. However, experimentalists like Thoang find it "very hard to carry out experimental work. You can carry out only part of the work, requiring not very complicated facilities." The visit by the two Vietnamese scientists to the U.S. signals open-
ing of "a new period in Vietnam history and Vietnam science," Hieu says. A new> more pragmatic generation is coming into power and Hanoi has begun moving in a direction echoing the Soviet Union's perestroïka (restructuring), reducing central control of the economy and opening up to foreign exchanges and investment. Vietnam is already sending several hundred doctoral students a year to the U.S.S.R. And a few hundred postdoctoral students a year go abroad-—to West Germany, France, Italy, Japan, the U.K., the Netherlands, and Sweden, as well as the U.S.S.R. Indications are that the U.S. will not lift its economic embargo or establish diplomatic relations with Vietnam during the next year, as long as there is no Cambodian settlement acceptable to the U.S. and no resolution of bilateral humanitarian issues. This impedes scientific relations and prevents any funding from U.S. government agencies. However, significant progress is already being made in fostering exchanges, particularly by the nonprofit committee, founded in 1978, that Ladinsky heads. For example, she notes, the group sends seven td 10 tons a year of books and journals to Vietnamese libraries. Joint research projects, lecture tours, and visits under the group's auspices will send 40 U.S. scientists to Vietnam this year, while 70 Vietnamese scientists come to this country—including at least five chemists. Alvin H. Weiss, a professor of chemical e n g i n e e r i n g at Worcester Polytechnic Institute, for instance, now has a postdoctoral student, sent by Thoang, doing research in catalysis. And this year, the U.S. is letting several Vietnamese study here for M.S. and Ph.D. degrees. However, Hieu notes, if the U.S. normalizes relations with Vietnam and removes the embargo, his country hopes not only to expand scientific cooperation, but also to attract U.S. investments. Vietnam offers "good potential for natural resources"—with interest by U.S. firms in its rare earths, for example. It offers hydroelectric power, "rich scientific potential," and "very cheap labor," as well. Richard Seltzer November 6, 1989 C&EN
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