Boston Scientific Corp. swung to a first-quarter loss on restructuring charges and a write-down of the value of its defibrillator business. In January, the Natick company said it plans to cut 900 to 1,000 jobs this year. It lost $354 million, or 26 cents per share, in the first quarter. That compares with a profit of $113 million, or 8 cents, a year ago. Excluding charges, it would have earned 16 cents per share. Analysts polled by FactSet expected earnings per share of 9 cents.