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I want to convert R 'library:raster' 'extent' object to a shapefile. I have the CRS available too. 

  e=  c(4304916, 4305325, 365216, 365439)#xmin, xmax, ymin, ymax
  proj4string(e) = CRS("+proj=utm +zone=18 +ellps=GRS80 +datum=NAD83 +units=m +no_defs")

In older version of raster data there was a specific function "" 'extentTopolygon' which is depreciated now. My current approach is to create a raster object change its projection and assign the current extent and convert it to shapefile, which is not very efficient.

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You can do the following:

library(terra)
e <- ext( c(4304916, 4305325, 365216, 365439) )
p <- as.polygons(e)
crs(p) <- "+proj=utm +zone=18 +ellps=GRS80 +datum=NAD83 +units=m"
writeVector(p, 'file.shp')

Or with the raster package

library(raster)
e <- extent( c(4304916, 4305325, 365216, 365439) )
p <- as(e, 'SpatialPolygons')
crs(p) <- "+proj=utm +zone=18 +ellps=GRS80 +datum=NAD83 +units=m +no_defs"
shapefile(p, 'file.shp')
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    Just a comment--the coordinates are given are swapped. The four million values are the 'y' or northing values while the three hundred thousand values are the 'x' or easting values.
    – mkennedy
    Commented Sep 27, 2015 at 14:15
  • @mkennedy. How so? I do not think so. See plot(p, axes=TRUE)
    – Robert Hijmans
    Commented Sep 27, 2015 at 15:52
  • Okay, I don't see that in the question or answer (in the current versions), but UTM is always easting,northing so...? You know the software, not me.
    – mkennedy
    Commented Sep 27, 2015 at 16:42
  • right, and easting is x, and northing is y, hence the order
    – Robert Hijmans
    Commented Sep 27, 2015 at 20:10

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